Preferably both. Not sharing passwords is the obvious one, but if I stop using something I'd like to be able to delete everything related to me (except perhaps logfiles or whatever else needs to be retained for operational reasons up to a few weeks). And without having to call someone on the phone please or email support@ please.
Or getting rid of passwords as a mechanism entirely.
Not that I have a solution, just whining :)
Mozilla Persona, I will suggest.
You're right, though I don't share passwords between services, I'm curious just the same.