I listen to... a lot of stuff, like most people here, so I can go either way on the matter.
Listening to a lot of Post-Rock (Explosions In The Sky, GY!BE, A Silver Mt. Zion, most Mogwai), you start to forget why singers are necessary at all. Obviously, music can do without them (albiet the previously mentioned bands can get depressing with their lack of catchy hooks and their untraditional song structures).
If a band has a singer, lyrics are important for two reasons:
1) The singer's voice is an instrument. The words the writer of the song (who we should hope is the singer) chooses create sounds that tie themselves to the song and should add another degree of depth to the sound. In most cases, they are a center-piece, and so those words better make some damn good music.
2) What they sing is just as important as how they sing it. Even if the lyrics are nonsensical (see: Of Montreal, The Shins), they should still make sense within the idea that the song is presenting.
Personally, I love poetry, so I like it when an artist has skill in writing lyrics that both sound great both musically and in written form. I like to see words played with and used in creative ways. I really have a hard on for vocalists and songwriters.
McArtney/Lennon = incredible. Listening through Beatles' albums like Abbey Road, Sgt. Pepper's, Revolver, and the White Album completely blow my mind with the amount of incredible songs they were able to write with such great lyrics, beautiful harmonies and melodies, and ingenious song structures. I don't care if you're one of those reactionary type folks who hates the Beatles just cos...or if you don't enjoy them period....
McArtney and Lennon were musical geniuses when it came to songwriting. (and not to mention ol' George)