Most professional MMA fighters with a few years in have severe debilitating injuries. I am not talking about a bad knee of slipped disc, I am talking about severe chronic pain and brain damage, something that is the inevitable result of the kind of punishment they deal with. Just look at someone like George Saint-Pierre who suddenly forgets what he has been doing for the last six hours.
It doesn't matter if fighters participate of their own free will, I could probably go into the slums right now and get two people to fight to the death over a $50 bill. There's a reason we have laws to prevent exploitation and don't just leave it to people's "free will".
I very much like MMA as a sport, but I wish there was a way to protect the fighters better. Headgear? Who can honestly say that they enjoy a fight where the fighters are bathed in each other's blood, not to mention the health implications (even the ref wears gloves).