The narrative at Mark 2:23-38 may be a polemic interpolation.
What Jehovah's Witnesses do not like to admit is that the Gospel preached by the original Christian community was totally presented by word of mouth. The written accounts, which came much later, each presented the Gospel with a slant designed for a particular audience and set of circumstances.
Mark wrote his gospel account at the time near the fall of the Second Temple (70 C.E.). Not only was there tension between the Jews and the Roman government, there was quite a bit of tension between Jews and Christians too, especially with the Church starting to become more of a Gentile-rich religion and less a Jewish sect.
Controversies in the written Gospels often have anachronisms which make them easy to spot as not belonging to the original oral traditions about Jesus. They seem to use the original oracles (sayings) of Jesus, but they transport them into new situation to give them a relatively "modern" import, modern to the time of the written compositions that is.
Take for instance this story of plucking grain on the Sabbath from Mark 2. The Pharisees are pictured as petty, nitpicking opposers to Jesus (as usual), but it is out of character for what is really a legal challenge (i.e., a concern over right behavior regarding Torah/Sabbath observance). Why would a valid concern, especially one important even to Christian Jews (see Acts 21:20 and compare to Matthew 5:17-19), be coming from the mouths of mean-spirited, hypocritical religious teachers?
The Pharisees here are a caricature, not the real thing. If what was describe really happened on the Sabbath, Pharisees would not be out in the fields to just stand around and make sure people were not working in them. Jesus' own citation of the David/bread-of-Presence story sets a precedence that was already well-taught in Judaism (even in Jesus' day) that human need can indeed negate the observance of any Sabbath requirement. Why would Jesus be teaching something that actually comes from the Pharisees (and using it as an argument against them)?
The conflict appears to be a narrative device to define the position the Christians were taking to the Jews during the Great Revolt. Whether or not the Flight to Pella actually occurred as Eusebius reported it, it is clear that the Gentile Christians were not going to side with Judean Zealots even if the Jewish Christians felt a need to. Whatever was happening within Jewish Christianity at the time, the need for lines to be drawn by leaders was definitely a need that could be served well by a written account of Jesus.
The conversation in the narrative in Mark 2 seems to be quite unconcerned with the details of what is being discussed. Because of the problems in the writing, as Crazyguy points out, some scholars have suggested that Marcan authorship connected with Peter is not possible. Why would a Jew confuse Ahimelech for Abiathar, not to mention the odd situation of the Pharisees being out in a field on Shabbat (which would give the impression that they were working, thus breaking the Sabbath, something that would obviously not occur in the slightest).
Yet, there is just enough of an argument to make for a true Marcan authorship possibility, or at least for Jewish Christian influence. The lack of precise details is also a Jewish narrative device, often used to get the reader to look beyond the report as historical to consider a moral lesson. If this is the case, it would make sense that the conversation is not the teaching material here.
Jesus makes the statement in Mark 2:25, "have you never read...?" but the details of the story Jesus is referring to are then given incorrectly. Mark writes that David "and his companions" were hungry and in need of food and that David "entered the house of God" when "Abiathar was high priest."
Not only was Abiathar not the high priest in the account being mentioned (1 Samuel 21:1-6), David acts alone, does not act out of hunger, and never enters the "house of God" to eat the holy bread. The idea that the author would get all this wrong after specifically having Jesus asked if the account had ever been read makes this very odd writing.
That Jesus likely taught such a thing about the Sabbath is true, as this was a common teaching in Judaism at the time and remains one today. It is actually the basic rule one learns as to how to properly observe Shabbat, as one must always refrain from doing anything for the sake of Sabbath observance when there is true, dire human need at stake.
Therefore the story must have placed Jesus' teaching in a tableau to illustrate the growing dislike Christians were developing for Jews in general. Teaching Christians how to view the Jews was a new lesson of the Church, one that did not come from the original oral teaching of the Gospel which originated with the Jewish Christians. With the Great Revolt in the air (or the dust from the Temple's fall still in the air if Mark was written after the Temple fell), it is likely that this and other "conflicts" between Jesus and "the Jews" were colored by political differences, not theological.
