He was burned over a huge percentage of his body. I don't know much about third degree burns, but I received second degree burns over my entire thigh and abdomen a couple months ago and the skin is still not completely healed. I can only now walk without pain. So I imagine third degree burns with skin grafts would make stretching difficult. Of course, you are totally right about him not being at as much of a disadvantage physically because he was in excellent shape and had no serious mobility issues. The thing about JR is that while he was a "challenge" contestant, he at least had some modicum of fame, since he was an actor on an ABC soap (RIP All My Children). I knew who he was because that was My Story, dammit (I'm still bitter, sorry). I honestly don't know who some of the challenge people are any more. It's Dancing with the Stars, not Dancing with the Inspirational.
That said, I loved JR. He had such a joyous perspective.