The Rousey catch has three things going for it.
1. She's actually hot now and it's not a case of getting a MMA star five years after they're a draw like TNA does.
2. She's only interacting with Steph so it's not going to make their own talent look bush league. And Steph stood up to her exactly like she would Sting or someone else, at least for the most part. Having her around doesn't make, let's say, Sasha or AJ look worse so long as it's only Steph that's getting the focus.
3. She's an honest fan, and not just some MMA mercenary, which helps.
So it has a lot of the positives of using someone with crossover appeal like this without the usual drawbacks. It didn't need 25 minutes at Mania or whatever, especially if it doesn't build to a match (and who knows if it will). I don't think her being there as an unannounced attraction made Mania feel more special, not as part of an endless talk segment. If it leads to a match, then it's worth it. If it doesn't, then it's even worse than bringing a part timer back to put over someone who's around all the time, because at least a part timer is an organic part of wrestling and wrestling's past and something that wrestling can claim credit for and not someone whose existence, when pushed as a big deal, sort of makes their roster look insignificant.