Actually when soldiers make their own decisions they DO become dangerous to their fellow soldiers. Soldiers need to be able to take orders and follow them to the letter. Doing so saves lives; their own and others.
As for the reason why D&D players are frowned upon, I think it has more to do with the lack of social skills, and a physique that is the direct result of little to no excercise and a strict diet of Cheetos and Mountain Dew.
After all, if you were stuck in a foxhole under enemy fire, would you rather have a massive muscle bound steak-head who lives for physical violence next to you, or some dude who is going to roll some multi-sided dice to determine how much damage he's going to do to Abdul the Terrorist with his Terrorist Slating Knife (which has +12 against Terrorists)?
/me wonders who will be the first to name what I've referenced several times in this post.