This could leak information to the other party if bailing is still available during the event. By trying to bail myself after you show up I could "prove" you really didn't want to be here after all.
Similar to the problem with the 'secret crush' version of the same idea -- people can insincerely tick every box, simply to see which of their friends has a crush on them. I guess the same caveat applies: only use this app with people you trust. (Of course, some would say that if you really trust each other you don't need this app; but I don't know, I can see a use case for those situations where your plans have become inconvenient, but not sufficiently inconvenient to back out at short notice without seeming rude. I wouldn't use it myself but I don't even use a calendar app.)