I have a homemade pack cover, no drain hole. I originally used it with an Osprey but it works with my Granite Gear and Mariposa Plus packs.
In a sense, they are useless for rain covers. I used mine crossing a bridge in front of Wapama Falls in Yosemite - if you are not familiar with that falls, in spring it will drench you to the skin and nearly knock you off the bridge with the force and amount of water. It did keep my pack dry, mostly, but water gets in around the edges and wets out the straps. Subsequent trips I took a poncho, which works better.
BUT. It is good to keep the mesh of the Mariposa Plus from snagging on things. It carries gallons of water to douse a fire. It makes a good ground cloth to spread gear out on. I can wrap a pack in it and hang it from the suspension of my hammock. I've used it to put dirty shoes in a tent to keep them out of the dew. I've carried a ton of firewood in it. In theory, I could find a depression in the ground and use it as a bathtub. I've used it as a gear hammock alongside my hammock. Since it's a bright blue, I could probably use it to signal someone. There are a ton of uses. All for a couple ounces of silnylon that I can jam into the bottom of a pack side pocket.