7 And he called the twelve and began to send them out two by two, and gave them authority over the unclean spirits. 8 He charged them to take nothing for their journey except a staff---no bread, no bag, no money in their belts---9 but to wear sandals and not put on two tunics. 10 And he said to them, "Whenever you enter a house, stay there until you depart from there. 11 And if any place will not receive you and they will not listen to you, when you leave, shake off the dust that is on your feet as a testimony against them." 12 So they went out and proclaimed that people should repent. 13 And they cast out many demons and anointed with oil many who were sick and healed them.
I have little doubt that many modern Christians identify the most important message in this passage, and also its Synoptic counterparts, as a reminder to distinguish between those who are receptive to the Gospel and those who are not, in order that evangelists not waste their efforts on the wicked opposition of skeptics. As Jesus said, "Do not give dogs what is holy, and do not throw your pearls before pigs, lest they trample them underfoot and turn to attack you" (Mt 7:6). Now, I agree that this seems to have been a key motivator for Mark, and more so for Matthew and Luke (or perhaps their source document Q), for whom Jesus adds, regarding each population which rejects his disciples, "Truly, I say to you, it will be more bearable on the day of judgment for the land of Sodom and Gomorrah than for that town" (Mt 10:15). So, clearly, these ancient Christian authors shared in some sense the modern concern for resistance to their religion.
However, this was by no means the only object of their attentions. Consider that they discussed a mode of operation for evangelists which appears to have continued well into the day of the authors. For, we must consider another passage from the Didache, a late-first-century or early-second-century Christian document which outlines a code of conduct for both wandering prophets as well as their hosts (chap. 12):
1 But let every one that cometh in the name of the Lord be received, and afterward ye shall prove and know him; for ye shall have understanding right and left. 2 If he who cometh is a wayfarer, assist him as far as ye are able; but he shall not remain with you, except for two or three days, if need be. 3 But if he willeth to abide with you, being an artisan, let him work and eat; but if he hath no trade, 4 according to your understanding see to it that, as a Christian, he shall not live with you idle. 5 But if he willeth not to do, he is a Christ-monger. Watch that ye keep aloof from such.
Thus we consider a particular perspective when reading the Synoptic Gospels, in light of the Didache.
It must have been tempting for Christians to adopt an occupation whereby they could expect their fellow believers to provide for them food and shelter, and where their duties might not far exceed simply preaching the Gospel with alacrity. That both Mark and the Didache are concerned with the conduct of such men seems fairly obvious; yet they address, I think, slightly different problems arising from that same issue.
Mark specifies that a prophet must not hop from dwelling to dwelling (6:10), and Luke, perhaps quoting Q, doubly so (9:4, 10:7). What might be their intent, then? Although I cannot appeal to hard evidence to answer that question, I suspect the following as an explanation: It could have been that evangelists unhappy with the hospitality of a particular host sought to be welcomed by more generous neighbors. In other words, they might have felt tempted to troll a Christian community for credulous or naive benefactors. They may not even have done so consciously; for it is not the case that an irresponsible person must always be aware of the insidious and resource-exhausting character of his own actions. Whether or not they recognized the cast of their hypothetical tactics, however, the consequences surely would have been clear to their victims' communities, prompting the sort of disapproval we see in Mark and Luke.
The Didache, on the other hand, demands an itinerant lifestyle for evangelists. While Mark seems to take for granted the homelessness of evangelists, the Didache makes it an explicit requirement, and sets strict limits on the time such men may stay in any one place. Again, I must admit we have no external indication of the author's motivation, but we may yet propose a plausible explanation, namely that evangelists in that day tended to settle wherever comforts abounded. That is, we have the same hypothetical situation I have suggested Mark was addressing, only on a larger scale: Instead of going from house to house, a Christian evangelist could travel from community to community, until such time that he discovered hosts who would treat him relatively richly. The Didache, then, we may interpret as abhorring that sort of opportunistic behavior.
With all of this in mind, we ask if it might be possible to use these interpretations to inform a date for any of Mark, the Didache or Q. To this end, we may construct an argument that all three compositions date from approximately the same time, since we may read them to reflect the same or quite similar historical circumstances, in which wandering prophets have become a public concern. In my judgment, we ought to hold onto the majority dating of Mark's Gospel, scholars usually placing it c. 65-70 AD. If Luke quotes Q in 10:7, then under the hypothesis presented here, Q and the Didache both fall in or about this same span.
Now, certainly this argument of mine hasn't the strength to settle any dating disputes with confidence. In fact, at the moment it amounts at most to a plausible suggestion, relying not on the texts themselves, but on particular exegeses thereof, which, although I find them quite natural and believable, remain highly interpretative nonetheless. We must, then, retains serious reservations while seeking corroborating evidence. Alternatively, we may attempt to overturn my relative dating hypothesis with a more attractive explanation.
Of course, one may point out that evangelists clearly had been active and highly mobile since the earliest days of the Christian movement, even to the 30s AD, just after Jesus' crucifixion. The Acts of the Apostles and other ancient sources record the names of several traveling preachers, including Barnabas, Apollos, Titus, Timothy, Peter and James, among others. We can reasonably expect imitators of these early Church founders and authorities to have arisen fairly quickly. Yet it is not the mere activity of such evangelists which I suspect had occasioned the stipulations elucidated in Mark and the Didache, but rather an ongoing and publicly-recognized problem with such men taking undue advantage of Church hospitality. The key element, then, is this identification of wandering prophets as undesirable characters who sap Church resources, a notion we may not see in writings from the 50s and earlier, nor after the late 70s. If we find this idea expressed in an ancient document, then by my hypothesis it dates from or near 65-70 AD, or else the supporting argument breaks down.
One might interpret 2 Thessalonians to meet this very criterion. In chap. 3, vv.7-8, Paul writes to those Christians of Thessalonica, "you yourselves know how you ought to imitate us, because we were not idle when we were with you, nor did we eat anyone’s bread without paying for it" (ESV). So, if we regard the epistle as genuine, which I for one am inclined to do, we have at least one instance of instruction regarding evangelists dating from the 50s or early 60s AD. However, it's quite possible that 2 Thessalonians is actually pseudepigraphical. In either case, we are free to place 2 Thessalonians in roughly the same proposed period as Mark, Q and the Didache, c.63-64 if it is genuine, and c.65-70 if not.
Dating issues aside, one other consequence of reading Mark as I have is that we best understand Jesus' words as somewhat anachronistic. For, he is speaking, we might think, to a situation not in his own day, but in Mark's. On the other hand, the assumption that the passage has heightened relevance to Mark's situation does not strictly imply the denial of its historicity; it could be, for instance, that Christians of the 60s and 70s appealed to Jesus' charge to evangelize in order to justify their opportunistic lifestyle, and that Mark and Q responded by pointing out that Jesus condemned such behavior in that very charge. So, it is their emphasis, and not their content, which I suggest relates to their motivation. Still, I do consider it simplier to posit that at least the words "whenever you enter a house, stay there until you depart from there" (Mk 6:10), are apocryphal. Moreover, we ought to notice that it undercuts my argument for relative dating to suppose they are genuine.
The approaches I've discussed may already have some precedent in the scholarly literature, either supported or opposed, although if so I am unaware of it at the present time. In any case, further exploration of the issues seems prudent. Meanwhile, I must remind my readers that these speculations of mine likely have little real use. I do not regard them as constituting some kind of essential insight; for they are surely nothing so grand. I only offer them up for whatever value, however minimal, they may happen to have.
 Tr. Isaac H. Hall and Mr. John T. Napier, ed. Alexander Roberts and James Donaldson. http://www.ccel.org/ccel/schaff/anf07.viii.iii.xii.html