I disagree. They are great athletes because they can do so many things great that most football players can't. Playing other sports is a way to keep those diverse skills honed in ways that you won't get by just sticking to the football field.
There's just as great a chance for them to get hurt in the gym as there is in a friendly basketball game. Both the gym and other sports prepare them for on the field. Why are we going to bubble wrap them for one and not the other?
When I was in college, I played fall soccer. Not because I loved soccer, but because it gave me a fitness and speed advantage when basketball season came around. These guys are doing the same thing. They are keeping their cardio training up, getting lots of hand/eye practices, and plenty of agility training. Stuff they can't get in the gym.
Let athletes be athletes, because that's what makes them great. Sure there's a chance of them getting hurt, but it's no more than doing some of the other training. Heck, QB's get hurt walking in their driveway.