I have a lot of respect for bass players and drummers who turn out to be fine composers, considering that they play instruments that are not associated with melody lines.
I started playing bass about two years ago. I initially got into it because I considered it an idiot's instrument that anyone could play and it would compliment the skills I have in other idiotic instruments. I also wanted to sound like Peter Hook.
After studying it for a while though I realized I was wrong. Its true that anyone can play bass if you show them how, but after learning to play a bunch of songs I starting to listen really close to every song for the bass line. What I've learned is that bass is HUGELY important to song writing. Its not just a tone that plays in part with the guitar. Honestly, a lot of times its the other way around. Take a band like Iron Maiden, for example. Their bass player is possibly he most important member of the band. He writes almost all the music and he's the only guy (I think) that has been in the band uninterrupted since its creation. Honestly...even though we all love Maiden for Bruce Dickinson (we do all love Maiden, right?) Steve Harris is more important because the band and its songs wouldn't even exist without him.
Another band I like, The Killers (so sue me) is heavily bass dependent. Many of their songs pretty much
are the basslines. The guitar chords could just be synth chords added in post and do the same job.
Also, The Cure. They do a lot more with bass than most bands, playing New Order style with more FX. Lots of high notes, melody, etc. But even so the bass is absolutely integral and in this case as well the bassist is a huge part of making the music.
Now, if you want to start a Kiss tribute band (because we need more of those) then...the guy you know with the least musical talent? Give him the bass. Since he's not writing anything and we're only talking about Kiss songs (Kiss music is shit, btw) his job will be really easy. Anyone can do that as long as they have two functioning arms. The bassist's role in a real band though is really f*cking important. Once the drums and bass are there, the rhythm is established, the key is established, the guitarist...all that's really required of him is to complete the last %15 of a jigsaw puzzle.
Unfortunately, as with many things, its really hard to convince people of this until they've done it themselves, and at that point it doesn't work because when a bass player says that the bass player is crucial, it just sounds like a guy defending his useless station in life.