Most scholars seem to think the instructions on what they could and could not bring were symbolic (that is, they were done for the benefit of those who would see the apostles, and they would find some meaning in what they saw). John P. Meier, for example, writes,
Such renunciation of the ordinary equipment for a journey was meant as a prophetic symbol of the urgent, eschatological, life-or-death nature of this brief mission, as well as a symbol of the disciples’ total dependence on the God who was beginning to regather Israel through them.3I kind of wonder about this, though. Would the people in the villages they visited really interpret it that way? I'm not convinced Jesus was primarily concerned about how they would be seen by others. I think it was done not for the benefit of outside observers, but for the apostles themselves.
This story actually reminded me of something early in my teaching career, when I was teaching religion in a Catholic high school. I brought a grade 11 or 12 class (I can't remember which) on a trip where we were taken on a tour to learn about the life of drug addicts, prostitutes, and the homeless. The tour guide had the students try something that was way out of their comfort zone: for an hour and a half or so, they had to pretend to be homeless and see how they would be treated by strangers.
Because they were doing this for such a short time, I don't think it was quite as eye-opening as the tour guide hoped it would be. But the purpose of the exercise was clear: he knew that in order to have any understanding of what it's like to depend entirely on the kindness of strangers—and endure hostility from some of them—you can't do better than actually trying it out for yourself.
And I can't help but wonder if that's what Jesus was doing to his disciples. If Jesus understood his vocation as, in part, bringing "good news to the poor" (cf. Luke 4.18), and if he expected his disciples to do the same, perhaps he wanted them to understand the people he was so concerned about.
Now, one might argue that his disciples were already poor. And in a sense, they were. But when we understand what "poor" means in the context of the gospels, we find that Jesus's disciples would not have been considered poor.
The Greek word translated "poor" in the gospels (in the NRSV) is ptōchos.4 This is not run-of-the-mill poverty; it's more like destitution.5 It originally meant "begging," and typically refers to those who are "dependent on others for support."6
The Twelve apostles were, as far as we know, gainfully employed before Jesus called them. They were not rich men, but they were not ptōchoi, either. So traveling in such a manner as described in today's reading would have been an educational experience: they would learn the hard way what it means to depend on strangers for support.
 Scholars often try to find some symbolism in the number two. Some find it likely that this "reflects the OT stipulation that two witnesses are required to establish legal testimony" (Marcus, Mark [AYB], 1.383). This strikes me as somewhat farfetched, since they are preaching a religious message, not testifying in legal setting. Morna Hooker admits as much, but defends this interpretation nevertheless, noting that "the idea that any kind of testimony needs corroboration seems a natural corollary" (The Gospel According to Saint Mark [BNTC], 155). I don't really buy it. Jesus, after all, preached on his own. And the disciples were merely relaying Jesus's message, not something they had "witnessed" themselves.
I think there were two because sending them individually would not have been as safe (Jesus knew his message could provoke anger, as we saw in last week's reading), and three would have been less efficient. This was recognized by the 11th century bishop Theophylact of Ohrid, who noted that if Jesus "had sent more than two, there would not have been a sufficient number to allow of their being sent to many villages" (Aquinas, Catena Aurea, 2.108-109).
 A number of scholars, notably John Dominic Crossan (The Historical Jesus, esp. 338-339), have argued that this reflects the influence of Cynic philosophers, but I find this unconvincing.
Basically I agree with Paul Rhodes Eddy ("Jesus as Diogenes? Reflections on the Cynic Jesus Thesis," JBL 115 (1996): 449-469), who finds the similarities between Jesus and the Cynics superficial and ultimately insignificant compared with the vast differences between them. He argues that Jesus is better understood within the Jewish wisdom tradition, in that his teaching is very Jewish in both content and form (especially his use of parables). The "Cynic theorists," however, reduce Jesus's Jewishness "to little more than an ethnic accident" (468). Also, whereas the Cynics prized self-sufficiency, Jesus's teachings "attest...to one's absolute dependence on God" (463). In addition, there is no evidence of a Cynic presence in Galilee during Jesus's time (463-467). A more compact argument against the Cynic hypothesis can be found in Donahue and Harrington, The Gospel of Mark (Sacra Pagina), 192-193.
Joel Marcus has suggested that, while there are some "overlaps" between Jesus's instructions and the practice of Cynic philosophers, "there are more extensive parallels with the Exodus tradition":
Presumably, their needs for food and shelter will be catered to by those who receive their message. But in a deeper sense the disciples will be looked after by God, and in this regard it is significant that there are strong overlaps between the Markan description of their equipment and OT portrayals of the gear of the Israelites whom God led out of Egypt and sustained in the desert for forty years. As the Markan missionaries will not need to bring bread with them, so the Israelites did not need to take bread because manna rained down on them from heaven (Exodus 16). As the Markan missionaries are enjoined to take only one suit of clothing, so the wandering Israelites did not need to replace their garments, since they were supernaturally preserved from deterioration. (Marcus, Mark [AYB], 1.389)Marcus thinks that Mark has deliberately inserted parallels to the Exodus story to suggest "that the disciples' missionary journey will be a participation in the new exodus inaugurated by Jesus" (Marcus, Mark [AYB], 1.389).
I find this possibility very intriguing, but I prefer to think about this a little more realistically, as if this is a story that might actually have happened.
As far as that goes, I think that there is likely some historical event underlying this story (see Meier, A Marginal Jew, 3.154-157, for a detailed argument), although it is clear the story as we find it here has been shaped by Mark for his own purposes.
 Meier, A Marginal Jew, 3.155.
 The only exception I can find is Luke 21.2, where "a poor widow" translates tina chēran penichran. But she's described as hē chēra hautē hē ptōchē ("this poor widow") in the very next verse.
 Crossan, The Historical Jesus, 271-274.
 "πτωχός," BDAG, 896.