I would never consider "hiding" anyone away, no matter what the handicap, but I can see some of the parents' logic behind their wish to have their children mainstreamed. Also, non-handicapped children benefit from spending time with disabled students -- it takes away the stigma of the handicap and gives the kids a better understanding of the many ways God made us.
But I feel sorry for the child who's not quite up to speed with his classmates. My concern is that this child will become the victim of a self-fulfilling prophecy -- he believes he's stupid because he's always the last one to finish. So he never develops the desire to try, because, no matter how hard he works, he comes in last. In a class with his peers, he would experience competition and success along with his failures. Personally, I think that's a healthier situation for mildly disabled children.