Strength of Schedule. With BYU not on their schedule, that leaves me ZERO reason to root for them.
And with 2 of Utah's opponents on our schedule (Oregon and Stanford), it would be silly to not root for Oregon and Stanford in those games.
Then there's recruiting. It's easier to beat Utah for recruits if they aren't playing very good football. So in that way, a winning Utah program is actually detrimental to BYU as it steals potential recruits away from them.
Again, this is all logical and not born out of an irrational hatred of Utah. But I just don't see a rational argument for a BYU fan to root for Utah to win all their games (or any gamea for that matter) this year.