As was mentioned earlier, bass is fundamentally important to pretty much all "Western" music styles, not just metal. I do agree that the bass is often buried in the mix entirely too much, though, particularly in metal. I also think that this is a crying shame, and any band suffering from this malady should immediately fire their producer!
The bassists works with the drummer, really more as one giant, cohesive instrument to define what is for me at least, the most important element of the music, the GROOVE. If you ain't got groove, you ain't got
shit in my book! How lame would Pink Floyd's "Money" sound without Roger Waters' unforgettable, pulsing groove driving the song? Can anyone here imagine Iron Maiden being half the band they are without the relentless galloping pocket driven by Steve Harris? Or shit, Opeth for that matter... both Mendez' & DeFarfalla's contributions to the band's epic sonic signature simply cannot be overstated. Even the decidedly more straightforward but
totally grooving walking basslines of a guy like Tommy Shannon (SRV & Double Trouble) are vital to what is essentially, the heartbeat of the song! And don't even get me started on funk... THERE IS NO FUNK without da bass inya face!
I've often found that a really sweet, grooving bassline will hook me on a song faster than anything, often without me even realizing at first what it is that's drawing me into the music. Conversely, music with a lame or undiscernable bassline will almost instantly turn me off, sounding completely lifeless and flat to me.
It really is a shame to me that bass players are often thought of as nothing more than failed guitarists, and that their vital contributions are so often underappreciated and disrespected. A great rhythm section will elevate an average band into a damned good one, and a shit rhythm section will instantly sink even the very best guitarist or singer.
Groove or die, bitches!