EInk displays only use power when they're changing what's on the screen, right? They work by giving a static charge to a bubble full of ink, and that charge should remain for a while, even if the screen loses power. So I don't see why screensavers are necessary.
You may be able to argue burn-in issues (I'm not sure if any are permanent, but they can definitely last quite a few black/white cycles), but that's about it AFAICT...
