That's the biggest problem I have with the sport specialization crowd: the cost. It's not a fair playing field. Someone who may have the talent but doesn't have the opportunities that another kid does gets left behind due to circumstances well beyond control. I think it's great that kids can play sports year-round. I think it's terrible that money has become such an important factor in it. The commercialization of youth sports is a HUGE problem. Too many people just using it to make money instead of developing/growing kids. That's one thing I like about sports in a local setting, at high school or within the same towns/communities: they're usually better at developing a person along with the athlete.
On a slight chance of topic, I think kids playing one sport year-round is probably not a good idea until high school due to the reasons others have mentioned, like burnout and the heightened potential for injuries. Once they're into high school? Have at it. But I don't think you have to play a sport year-round as a 10-year-old to be a stud in high school. Just be an athlete.