This is a good topic. I have often wondered myself if lifting weights to the degree some athletes do is really beneficial. I personally don't think that having a weight lifters body is really helpful. And it seems as though weight lifters (and people who look like them) suffer more injuries and actually die younger than their counterparts.
Of course nobody is saying its okay to be a fat slob, but I do believe that all of the "bulking up" is leading to more injuries in both the NBA and NFL. I also think there are a lot of really good posts in this thread showing that constantly adding muscle can actually limit pure athleticism.
I wonder if a player like Durant will succumb to the pressure of weight lifting and the need to "bulk up" as his career progresses. I actually think such weigh gain would hurt his play more than help.
Tiger Woods greatly bulked up later in his golfing career, adding more than 30 pounds of muscle to his originally skinny, athletic frame. Ironically, at the same time, he also suffers from far more injuries today than he did before he was involved with heavy weight lifting.