I don't think the Tony voters are as complicated and impenetrable as we like to make them seem. My thought on the Voting Mentality can be boiled down to one sentence: "I will vote for what I liked most, unless you give me a really good reason not to."
As for Best Original Score, I don't think SHREK is even on the radar: it'll be between NEXT TO NORMAL and BILLY ELLIOT (and it's a really close fight at this point). My opinion: BILLY ELLIOT will win, because a voter's impression of a score can be very very easily influenced by the physical production of the show, and BILLY's physical production for the numbers is superb.
(but I also think Adam Guettel's LIGHT IN THE PIAZZA score won because, even if the voters didn't enjoy it completely on a first listen, they were rewarding a New Generation Composer who had a whole lot of admirers.)
(not that it's not a wonderful score)
SHREK isn't on the radar because SHREK is not an easy show to admire, even if you like it: it's corporate, it's a machine, it's yet another Broadway Theme Park. And when you've got NEXT TO NORMAL and BILLY ELLIOT, which would you feel better about voting for?
(of course, some of this depends on whether a voter has actually heard all four nominated scores, and you can be sure that some voters haven't.)
"Y'know, I think Bertolt Brecht was rolling in his grave."
-Nellie McKay on the 2006 Broadway production of The Threepenny Opera, in which she played Polly Peachum