The reason why no one ever listens to the bassist...
...is the dreaded rhythm guitar.
Okay, so it's not entirely the rhythm guitar's fault, but it's a major part. Other factors include moronic producers at studios who turn the bass levels down so low that it becomes barely audible, and less than significant. Which is less of a problem in live situations, because like skongshoj said, sound crews usually have their shit together. That's a write-up for another day, though.
First of all, I don't want to blame the rhythm guitarists themselves, they're usually great guys and are just fulfilling their duties. Instead, it's the concept of a rhythm guitar itself that I blame. The rhythm section is traditionally a role filled by the drums and the bass. Then, the melody is taken care of by the guitar, keyboards, or whatever it may be. Ergo, the rhythm and melody work in perfect harmony and create musical bliss.
Then comes the rhythm guitar. Usually, the rhythm guitarist plays chords to provide a backing for the lead guitarist, who plays melodic jingles and blistering solos. Often, the rhythm guitarist uses distortion for these chords, effectively creating a wall of sound. A wall which often blocks the bass from being heard. This is what destroys the harmony of the rhythm/melody zen. It fuses all the instruments into one undecipherable being, a constant flow of noise. Instead of each piece having its own voice, they're a single homogeneous mixture. And the poor bass gets drowned out.
Maybe you need that extra crunch that the chords provide to give your music a raw, heavy feel. Black Sabbath comes to mind, as they only used one guitar. Yet, were they not Heavy Metal? Geezer Butler did a phenomenal job of outlining everything guitarist Tony Iommi was shredding, or vice-versa, Geezer Butler provided an amazing palette which Tony could use to mold his guitar riffs. They didn't need a rhythm guitar, because they already had an extraordinary rhythm section.
Perhaps since no one ever listens to the bassist, having a guitar take its place would make the world a better place. I think not. The bass is also responsible for holding down the groove, something the rhythm guitar just can't do! Groove is something essential to a successful band. Why? It gets the ladies dancing! Any good bassist knows that even if you can't hear the bass, you should feel it. While people may be in awe of the guitarist's white hot shredding, it's that bass lick that you can feel through your bones that gets you up dancing.
Let's take Led Zeppelin, for instance. If you've ever heard a Zeppelin tune, chances are, you've heard John Paul Jones' groovy bass licks. If not, you've heard his mandolin-playing skills or innovative keyboarding. You may not have known it's the bass you were hearing, but you definitely felt it deep down in your soul. Dazed And Confused? That groove could make a grown man cry. A rhythm guitarist only would've dampened the sound and hindered their individual talents from being heard.
Now why in the world would you want to add someone to the band that's going to defer the bassist from performing his most vital role? Bassists may not get the most tail, but the rest of the band sure as hell owe their lives to him for making it possible.
Besides, there's one less person you have to split your earnings with after the show if you don't have a rhythm guitarist.