If I was hoping for a career as a professional athlete (you still want to do baseball, right?), and it wasn't going to be in BMX, I would definitely not take any unneccesary risks or push myself. That said, look at Revmonkey, he got ran into when he was cruising down a parking lot and ate **** for the last time.

I'm not going into a career where a less-than-100% leg would make it impossible, but I also don't have health insurance and my parents can't afford a big hospital bill, so I take it easy on the bike and know my limits. If you keep riding and trying stuff, but not way out of your ability, you'll eventually progress and get pretty good without getting hurt too badly. Honestly, brakeless is an unneccesary risk, but one I choose to take because I never go balls-out at anything.

If you think you can make it as a pro BMX rider, go for it, the injuries are a neccesary evil, but if not, you'll have to decide if being good at a hobby you have as a kid is worth settling for a career you didn't want later.