Reba's answer is probably closer to the truth but I would think too that it's a positive thing to have the child spend time with their family... though it's more of a positive if the family actually interacts and does sign with the kid. I've heard stories of kids who went to residential schools but when home the family never bothered to interact or attempt ASL with them.
As it is.. I know that with one of the NC schools for the deaf, the dormitory residents have been declining quite a bit over the years. Either bused in or families move closer anyway... or there are deaf programs closer to home.
I would think too there are exceptions- if the child is from another state, the school would allow them to stay in the dorms on the weekends; I don't know if there are kids from outside of the US who attend deaf schools here...???