I've changed my mind about pack covers; I too used to be inclined to the "don't bother" view, but now think that it depends on where and what time of year that you hike.

I always use a pack liner, but I had a cuben pack cover last year and overall found it worth carrying (quite low weight and bulk penalty for when not using it, and I used it a fair bit). Certainly a poncho is a great alternative, or a Packa, or, indeed, very often just a pack liner and no sort of pack cover.

Also skewing this is whether you carry, on a particular trip, things that have to be strapped to the outside of the pack --- snow shovel, ice axe, rolled up ccf sleeping pad, whatever. Pack cover doesn't co-exist well with bulky or pointy things strapped externally. Of course that applies to a poncho (or packa or parcho or whatever) as well.

Somewhat related is just having easier access to things; messing with the pack cover is just one more step to getting something out or putting it away.