I understand your point about combining different educational needs in the same program, but if I taught a class that combined those two groups, it would be so much better for those groups of students than the almost completely heterogenous classes I currently teach.

Some distinct advantages I can see: '
work habits and goals of the highly motivated rubbing off on underachievers (positive peer pressure)
a group whose combination of characteristics lets them take similar leaps in achievement
larger group of peers in the ballpark of intellectual compatibility

As a PG underachiever, I would not have wanted to be cut off from the creative types or the high-task-commitment types, even if I did have a significant need for acceleration. The emotional and social need for peers would have trumped the intellectual challenge as far as I was concerned in middle school and now, in hindsight.

Of course, I am not a parent, focused on the needs of an individual student. I am about to start teaching 134 new students ranging from mentally retarded or emotionally disturbed to...who knows? My district doesn't really test for giftedness, though some criterion-referenced testing results suggest several students might qualify.