I don't think 2yos need to be in a serious, full- or half-day program for social development, though I don't think a well-run program harms them. By 4 or 5, if your child will be going to K, he or she should learn how to function in a classroom setting--wait turns, line up, be quiet when an adult is speaking, and get along and share with other children. (Well, even if not going to K, IMO--but it is not so necessary yet.) A 2yo can get some early social exposure in a lot of other ways, so no NEED for preschoo, but yes, find ways to socialize somehow, occasionally, anyway.
As someone who raised 2 very bright and in one case hyperverbal toddlers, I would say that they really benefited from being around other kids, even if at first they did not seem to. We learn from other humans, and from contact with other humans. Yes, even when some other kid is throwing a block at your kid's head. Of course, sometimes there is a poor fit, with another child or a teacher, or a poor environment.
