I agree with the status quo in this case, or else I'd have probably changed how it worked some time ago.

If you've ever been seriously ill, you've probably experienced a certain amount of lingering weakness even after the illness itself is gone. That's what I see negative mana/move as.

Even hypothetically, if plague stopped sapping at 0 mana/move, it'd probably have to be nearly uncurable to be as good. If you're not a Rager with no cabal powers you don't really have a legitimate bitch about not being able to deal with plague in most circumstances.