I'm against it in MMA and Boxing just because they punch and kick each other in the head. When Vitor Belfort was juiced up he was knocking people out stiff fight after fight, when he was clean and deflated he was pillow fisted pretty much over night.
In baseball, I would let them juice all day, they're just hitting a ball with a bat, they ain't handing out concussions based on their raw power.
Even those cyclists like Lance Armstrong always getting caught, I'm all for them doing it, they just ride bikes.
Personally I would keep it out of contact sports like fighting and football though.