Interesting point. I'd say that I really understand the lyrics in less than half of the music I've ever heard. I find it almost impossible to understand most lyrics in rock, and even the tiny bit of hip-hop I've heard has lyrics so fast that I can't possibly understand them. Even in the rock stuff that I like, I probably miss around half the lyrics. This has been the case since I really started listening to music around age 10.
The exception would be folk music. This week PBS had one of their fund-raising specials focused on folks music. Peter, Paul & Mary, the Kingston Trio, Pete Seeger, Judy Collins, that kind of stuff. Great music, and the emphasis is actually on HEARING the lyrics.
I listen mainly for melody and "hooks" that stick with me. That's probably why I dislike hip-hop, because there's usually little or no melody and the lyrics are machine-gunned so fast that I can't get them. When I see an interesting tune in this thread, 9 times out of 10 I have to look up the lyrics online to see what it said. You would think that if the lyrics actually matter to the band, they'd make more of an effort to mix their music so you can understand them.