"it won't matter whose base Thompson eats into..."
Obviously it matters a great deal. How could it not? If Thompson is more of a threat to the others than he is to Rudy how does that hurt Rudy?
Given the large field the winner of many state primaries could poll in the 30s. Now I do expect Hunter and some of the other marginal candidates to drop but if Romney, McCain, Giuliani, Newt, and/or Thompson are in to the last that will assure a Giuliani victory since the others are going after the same voters.
All Thompson needs to do is win a plurality in enough states to win a majority of delegates to nominate. If Giuliani continues to track in the low 30 percentile, Thompson could take votes from McCain, Romney and yes, even Rudy and Hunter, to gain enough to win enough primaries.
I about 6 or 7 months, we will have a better handle on this. Right now is still far too early. Ask Howard Dean about early "insurmountable" leads.