I remember this same issue being discussed on BN about 10 years ago, and I think it was MSU Toddler who noted that he knew very few fans who actually attended MSU or who actually attended UM who would root for the other when their school wasn't playing. His position was that Griz fans or Cat fans who did not attend either school didn't have that instilled hatred of the other, and would root for the other when their team wasn't playing. Over the years I've thought about what he said, and at least in my limited world his perception seems to be pretty accurate.