A lot of times, I'll get an album because I like one song I've heard from it, or because I like other albums from the same artist, and then even if I don't, right now, like most of what I got, there's no reason to actually throw away the ones I don't. Even if I never end up liking those other songs, at least having them will keep me from wasting time getting them again in the future when I notice the hole in my collection or album and wonder if, just maybe, I'd like those other songs...

Disk space is cheap enough, in other words, that it's easier to just keep songs than keep an updated list of songs that you don't want.