Sometimes candidates who have no real hope of winning stay in a race in order to bring their issues to the agenda and to shape the debate. Agenda control is a very important part of the political process. Ron Paul talks about things that others don't, so he brings those issues to the table that might otherwise be ignored. I don't support Ron Paul, don't agree with him on most things, and I knew from the beginning he had no chance, but I think he really believes what he says, and I think he's staying in becasue he wants his issues heard as long as possible.

Another possibility is that he will make a third party run, and he just is using the Republican primary as a way to get some publicity before dropping out of the party. A third party run would have no chance either, but he might do that for agenda-control reasons as well.