Eh. All I can say is, I've never been watching a fight and thought "Man, I'd really like to see that again. But this time from a much lower definition and with a shakier view.".
But, I don't really like replays at all, during a fight. I'd much rather see/hear what's happening in the corners between rounds.
Even if you only use it for questionable stoppages, I doubt the usefulness of it. It would only show what the ref could have seen, not what he was looking at, or focusing on. They have enough cameras around the cage that we can get that anyway. Not worth the explanation of why all the refs wear goofy glasses and backpacks to a friend I'm trying to get into the sport.