The writers thought up Drusilla before they thought up the PTB, so I don't think they (the PTB) have any relevance to this discussion. As I understand it, Drusilla was precognitive, but in the Catholic tradition, as Dru's mother told her, only God is supposed to know the future (and then there's the whole "thou shalt not suffer a witch to live" and stuff). I think the visions are part of her brain, part that won't go away just because she's dead. They, in fact, became as perverted as she was, showing her things that helped them destroy the world rather than things to help protect people (like the mine cave-in she saw before she died). The only indication in the Buffyverse that God exists is in the people that believe in him, not in the mythos itself, so I don't see why the visions would disappear just because Dru's an "evil thing."
I hope that made sense; I pretty much wrote it as I thought it.