I think it has more to do with fears about the self-esteem of the kids not in that class, as well as the crankiness of parents and/or teachers who think fair = having the whole Bell Curve represented in every class.
Even if they put the top 5% in a class, it would work better than it does now...
Or there's the grouping that doesn't try to fill the entire class with GT kids, but has various groupings in a given class. The big caveat about it, as I recall, was that the GT kids shouldn't be with the lowest performers nor with the top-but-not-GT group. GT kids hampered the performance of both those groups.
