Actually, a win by Paul might give him momentum to win a few other caucuses (the Dakotas, Alaska, Montana, and Washington are his best shots), and at least win delegates in some other states.
A win by Santorum might enable him to briefly emerge as the anti-Romney. My sense at this point is that if Santorum, Newt, and Paul can all stay in long enough, this thing might still go to the convention.
This. The reason a win in Maine or anywhere else by Paul is relevant is because he's on the scoreboard that matters. Right now, he's a really succesful boutique candidate. Santorum is, conversely, an unsuccessful mainstream candidate. That said, and before Phil jumps all over me with various Paul-oriented epithets (I'm not nearly as big of a Paul supporter as Phil might think, by the way), a win by Santorum in any state will give him new momentum. I think Gingrich will be very hard-pressed to stay in after a Santorum state win, his ego notwithstanding.