"If you guys know that aikido does not work in a fight and in training do not really give your partner a convincing attack and then just roll or fall down, why do you do it?"
All uses of violence aren't the same, they run the range from brushing aside some drunk teenager, who's just a little exuberant and decides to take a swing at you, through to someone lying in ambush and blowing you away with a trap of some sort. I want something to disable people who are very little threat to me when I can't, or would feel guilty about, smashing their face in. I already know how to win fights at far more extreme levels of violence - should the situation go that far. If aikido fails, so be it, no biggy. But I've used it to wrap up and drop people who I'd certainly have felt guilty about hitting and my conscience is somewhat easier because of it.