I think an activity can be a good way to teach kids about discipline, teamwork and sportsmanship....all important lessons, but the key is to balance it with everything else. My son plays chess...he started with soccer, but he wasn't very good at it and the coaches were so focused on winning that DS never got to play and he was constantly the scapegoat during practices....the coaches would literally set him up for failure to make him run, probably in the hope of motivating him to improve. But the kid just doesn't have the motorskills and coordination for sports that involve a ball. (we're starting physical therapy again in a couple of weeks to work on the motorskills) He came home in tears every time. So we switched to chess.
DD does musical theater. She doesn't have an athletic bone in her body and gets extra help in PE because it's her weakest subject. So sports aren't her forte and she'd rather do musical theater. She still gets the lessons of teamwork and discipline.
But it's just that for both of them...a hobby. Sometimes they have to miss a lesson because of something at school, or whatever...there's nothing riding on it. Actually, DD took a whole year off because she was so incredibly stressed just with school that she would burst into tears, feeling like she had too much to do...at 10. So she took a year off, realized how much she loved it and missed it, and went back the next year.
That being said, some kids just LOVE everything sports. DS would have done everything if we had let him, but we just didn't have the time to devote to that many sports. He LOVES basketball, and baseball, and soccer, and he did every kids run there was...he's not GOOD at sports, but he enjoys them. (Well, he hates kickboxing, but so far that's the first one we've encountered that he didn't like) But the emphasis here, by his age (10) is on winning...if you're not good, you won't get to play. I'm thinking about the G team for soccer....it's for kids with special needs. It's all kids with Autism, downs syndrome, my neighbor boy played on the G team because he has some bone growth issue. So I've thought about that for DS to get some exercise and such, and I've thought about Judo to learn some self confidence. But some kids just want to do it all and as long as they are keeping up with school and other responsibilities, great. I just don't like it being so cut-throat at elementary school level. Let them be kids and have fun, you know?