There seems to be this assumption that there is this ideal of a good riff, or a good song, that is just out there in the ether, totally independent of any extra-musical considerations. But I honestly don't even know what that ideal is supposed to be, and I really don't know how people can go on talking about a well-written song without considering what possible meaning or emotional significance that song can have for them. Why is the song good independently of any considerations beyond its sound, texture, etc.? I can't even begin to conceive of an answer to that question because my experience of music has always been wrapped up in meaning and emotional significance. Ask yourself why you were first attracted to metal music. How would you explain that attraction? Maybe you thought something simple, like the stuff sounded heavy, or dark, or whatever, and you liked that. But those are not purely sonic notions. Those are concepts imported from ordinary language, and discussion of music is rife with these sorts of metaphors. My hypothesis for why such talk seems apt is that music isn't just something that sounds cool to us and that we all pretty much have an idea of music as an art form no matter how hard it is for some to articulate or how much some people want to deny it and reduce it all to "sounds cool, bro." So I already need extra-musical concepts, and interpretation, in order to place the music I listen to in a context, and to be able to say "This music does that sort of expression well."
Part of the value of lyrics to me is to add articulation to a musician's vision. Lyrics can act as touchstones that guide the listener toward understanding what the musician is doing. I want some idea of what's trying to be achieved with the album, what sort of thing the artist is trying to evoke. Album covers and song titles serve the same purpose for me. Of course, there doesn't need to be lyrics, and a lot of my favorite music is instrumental, but that doesn't entail that lyrics, imagery, etc. never matter. At any rate, as I've already implied, I don't think there is such a thing as truly absolute music. At least, there's no such thing in my experience.
From my own personal experience, the albums that I only appreciate for the "cool sounding riffs" or whatever are the albums that fall by the wayside and have no shelf life with me. The "it's just music, bro" attitude leads naturally to boring, ephemeral crap like Necrophagist. Think about it.