To a large part it's irrelevent imo. In Aikido we seem to spend an inordinate amount of time regaling each other with stories of what Ueshiba could do. What difference does that make if the modern generation can't replicate it? Surely we should be expecting subsequent generations to be delivering superior rather than inferior outcomes?
I agree that this seems to come up quite often. I not concerned with what Ueshiba could do. Not only that, I fall in the camp that believes Ueshiba, even in his prime, he would get his @$$ handed to him by today's MMA elite. Fighting has evolved far past what existed in Ueshiba's heyday. Not to mention many Japanese MAs tend to veer into self-congratulatory groupthink territory in regards to their effectiveness, founders, etc (IMO).
MMA/BJJ/"alive" training paradigms provide a forum for which a person can try things, see how they work, and measure them. As opposed to relying on supposition, hypotheticals, and hearsay about how a certain technique "should" work. It makes me think of the XKCD shirt "Science" found here: http://www.xkcd.com/store/