Socializing generally isn't productive for gifted children past a certain point; when they're too far ahead of their peers, there's not much value that comes from it.
Somewhat. If the gifted kids can socialize among themselves then that certainly is more beneficial. In theory it'd be nice to only interact with people like oneself but I don't see that happening, learning how to interact with others is definitely not useless but indeed depends on the amount. Not sure I'd measure usefulness with some productivity metrics, that success = productive kid at that point in time, social skills are hard to measure like that.
Children aren't really the "others" of the adult world; they aren't fully grown enough, generally, to be of interest. Were the child or adolescent interacting with non-gifted adults, it would make sense. Not so much children.