Among coaches, there are conflicting thoughts. Off-season work is important, but it's not the only factor. It makes them a better athlete, I think. They're probably going to be in better shape. And, by playing two or more sports, you have a coach who is constantly on you about grades.
Some coaches know about the benefits first-hand.
And a lot of coaches are concerned about injuries. But, at that age, I think it's still a time to find out what your real passion is. And I just think it's good for you. You meet new people, you're always doing something different. I know I was always looking for a challenge.