Changing the behaviour when sep is None is a big backwards-compatibility break, and I'm not sure we'd even want that. It's logical to allow passing None to mean the same thing as NULL (i.e. no arguments), and the behaviour in that case has been like that for... well, long enough that changing it isn't really feasible.

I agree with Barry here, especially since this is a completely opt-in feature, and existing behaviour isn't changed without the user's knowledge.