Not sure how many of these bands exist, but I do know of one my friend kind of forced on me and now I love it like...umm...a really loveable thing.
The band's called Apocalyptica, in case you haven't heard of them and wanna try 'em out, I would suggest Resurrection (mostly just cello and percussion) and Romance (a much lighter song, not sure what's used). If you've already heard them and hate 'em, feel free to say so.
(sorry, not sure what genre they fit into, they play different stuff)
From what I know of the band, they're a cover band.
I remember hearing about them playing Metallica covers with just cello's.
I've never really checked 'em out for fear of them completely destroying Metallica.
I'll see if I can pick up a CD or two in the upcoming months.
As far as "No Lyrics" go, there are a bunch of bands that don't use lyrics.
Explosions in the Sky
Godspeed You! Black Emperor
A Silver Mt. Zion(Minimal Lyrics, there's lyrics in some songs, but most of it is instrumental)
Basically a lot of post-rock stuff.
Bands like Pelican, Isis and 5ive have very minimal lyrics.
Pelican has none.
5ive has none I believe.
It's been a while since I've listened to either.
Anyways, my point is, the human voice isn't needed in bands.
It's certainly the weakest instrument of all.
However, it's a double edged sword because it can also be the one that adds the most depth and emotion to the song.
You do present a good point, lyrics are good, but sometimes they're bad (holy crap, that sounded stupid). Like, if your voice cracks like a boy going through puberty and you're 28, you shouldn't be singing.
It's been very rare that I have heard a cover and thought it was done very well.
By the way, if you're talking about that 311 cover of The Cure that came out with that terrible Adam Sandler movie I'm going to have to disagree with you. 311 can't even make their own songs sound good.