If it were single-user, that'd be one thing, but given that the whole shtick with Trello is that it's collaborative, you find really fast that there are almost zero real-world situations where you can actually do something sane when actions diverge. You'd end up building a DVCS into Trello just for cards, and then having a hilarious conflict-resolution process. That goes pretty strongly against Trello's KOSS ethic.

(I don't speak for the Trello team, but I remember trying to reason out whether we could do something similar on FogBugz, and rapidly concluding that the real answer was "absolutely not.")