Fellow sysadmin/developer/DBA here. I've been on the receiving end of bad merges that needed to be undone before and certainly understand your frustration, but the effects of an "undo" feature could be far-reaching.
I think everyone agrees that successive edits and merges after the "snapshot" would be affected by a rollback, but the big issue that I can see is that since profiles are part of a singular (well, one day maybe) collaborative tree, many users could be potentially affected. You may have one of your ancestors merged by another user and then later "un-merged" only to have subsequent edits by yourself and others just disappear into the ether. Granted, as IT folks, we know that there are ways to keep them from just being tossed, but I would imagine that the majority of the user base would be fairly alarmed if this scenario were played out, regardless of how the back-end system is designed.
Sure, something could probably be designed to accomplish this and not offend the sensibilities of most users, but is it worth the ROI? This would surely take a lot of time to develop, but how many accidental merges actually take place in a given period of time?