I agree with the idea that liners are more effective. I really must try one sometime; I've used pack covers for years with no real problems (caveat: I don't hike in the rain very often any more.) The fact that my current pack came with a cover, in it's own little zippered compartment, means that inertia has a lot to do with my decision to use a cover rather than a liner.

I don't really like the idea of using garbage bags as pack covers. They work, sure - but they also snag and rip (particularly where one cuts slits for the shoulder straps.) And when they do, they end up discarded along the trail - like empty soda pop cans, they seem to be just too heavy to carry out.