For, at the very least, one reason:
USC will never again face UCLA with the Rose Bowl on the line. At most, it will be for the division championship, and then when that team plays the North Division champion, that game will be for the Rose Bowl.
Is South Division Champion the same as the Rose Bowl? No. It is not.
And that's another reason I already miss the Pac-10. I sure hope Colorado and Utah are worth it.