You should take up the bass because it's fucking COOL
That, and the world needs more talented bassists. I once heard a rumour (skongshoj's uber-credible sources strike again) that for every bass player, there is something like six to eight guitarists. You have to know how to deal with rumours in the underground music world, and since a good part of them can be traced back in a straight line to their origin in the bedroom of some stoned freak in a Morbid Angel T-shirt, this usually involves a pinch of salt. Frequently, several kilograms of the stuff. Still, from my own experiences in the music scene, it could well have a ring of truth to it. On a related note, drummers, especially good ones, are even rarer. Should you ever find one of the good ones, have him surgically joined to your body while he's out cold, you'll be wanting to keep him at all costs. Since drummers have a well-earned reputation for knowing how to party, and since the drummer always gets laid, you'll be thankful for your new Siamese twin.
But I digress. The reason why so few people bother with an instrument like the bass is most likely that no one ever listens to the bassist anyway, so the Way of the Bass Player is a hard and sometimes depressing one to tread. It takes an even combination of balls, ego and a healthy dose of prescription self-loathing to even start on it. People remember their local guitar heroes and the shreddin' solos on their favourite records, but nobody ever notices that killer bass line. The reason for this is well within the realm of physics and psychoacoustics, an electric bass is a whole octave deeper than the guitar (and bass lines are frequently, but not always, played with the deeper notes of the instrument anyway), and the human ear has more difficulty noticing deep sounds than bright and high ones. To add insult to injury, most album producers (fortunately, this usually does not apply to stage sound crews at live shows; those folks usually have their shit together) are criminally brain dead and turn the bass so low that you'll be needing studio equipment or the Spider Sense to be able to hear that the thing is playing at all. Especially if there's two guitarists playing at the same time.
You won't be impressing girls with your monster bass lines, and there won't be a lot of people noticing you. When you tell someone that you play the bass, the usual response will be something along the lines of "my condolences, I didn't know". The legacy of Jimi Hendrix is that the bass player has all but disappeared from the minds of music listeners -- ironic, since Jimi's bassist Billy Cox was quite an artist in his own right.
Still, this is useful, since it means it's easier to sneak subliminal messages into our bass lines and TAKE OVER THE WORLD.
Eh?! So why the fuck SHOULD I take up the bass guitar?
First, you need to get rid of the habit of calling it a "bass guitar". It's a bass. If you want to call it more, call it an elecric bass, or even an axe. A bass guitar is basically a 4-stringed acoustic guitar with a bigger body and tuned lower, whereas an electric bass is more closely related to those big fucking wooden things you see in classical orchestras and jazz bands (it plays the same role in the lineup as those things. They're called double basses, contrabasses or upright basses). Besides, it usually has connotations of "a lesser guitar, played by people who can only handle four strings", and you don't want that now do you?
Second, your role in the band is a critical one. If you're any good, your band will love you. Seriously, expect marriage proposals. Your role is arguably even more important for the band than for its listeners, since you'll be the dude tying the music together. The drums are a pure rhythm instrument, and they will be laying down the pulse of your tunes. The lead instrument, whether it is a guitar, saxophone or even a bloody keyboard, will be handling the melody. Your task is to make a link between those two sections, and while rhythm section is usually what your job description will say, a good bass line relates closely to the melody. Much of the best heavy metal is composed by bassists and guitarists working together (every single Black Sabbath song until Geezer Butler's departure was "composed" by Geezer playing a bass line, and the others falling in when they figured out what to accompany it with), and the same applies to most rock. If people can't hear the bass, you can be sure that they'd notice if it wasn't there, in which case your music would be as boring as a World Wide Web Consortium Working Specification, and I'm reliably informed that therapists use those to put amphetamine wrecks to sleep.
The few listeners you'll get who actually listen to what you are playing are a special bunch; an elite breed among music aficionados. They're a minority, and they know it. Most of them are either bassists themselves, or they're long-standing music freaks who are capable of picking out whether the guitar has a bone or graphite nut through four layers of effects at a distance of two miles, with enough brain power and listening accuracy left to enjoy the groovin' bass line and notice that little blooper the drummer just made on his ride cymbal. You'll hate them to begin with, since they'll be pointing out your mistakes to you after the show. You will also love them, since they listen and they care. If you do well, you will become a god to these people, even more so than any guitar hero could ever dream of. You have ignored instrument points, and that's important to these people (you'll most likely notice yourself becoming one of them when you're on the other side of the stage). Don't count on picking up a million groupies, bassists are cursed by God, and when He created groupies, he equipped them with an extra sense that allowed them to notice the curse and steer clear.
A musical feature the bass has over the guitar in hard rock and heavy metal is the fact that most often, the guitars will be playing through heavy distortion to give them that cool, aggressive sound, while the bass is usually played dry (ie. without effects), or at most with a touch of chorus. This means you'll be playing "pure" notes, which can be used to a very good effect in combination with those heavy guitars. Listen to any Kyuss, Queens of the Stone Age, Tool or even good old Iron Maiden record to see what I mean. Some, usually black metal bassists, regard this as a bug, and slap a heavy distortion pedal onto their rig (fortunately, this means that they become patently impossible to hear).
Should you become a GOOD bass player
You've mastered the bass. Your preferred instrument is a six-string fretless, you tap like Stu Hamm, you slap like Les Claypool, you solo like Cliff Burton, you have the speed of Steve DiGiorgio and the stage attitude of Steve Harris. There's no such thing as a bass hero, but if there was, you would be it.
You still don't have a million fans like your band's guitar shredder does, and you don't pick up 1014 attractive and willing members of your preferred sex per weekend like the drummer and lead vocalist do. Fortunately, you don't care anymore. Having walked the Path of the Bassist to the Palace of Wisdom, you have become a paragon of quiet self-confidence, and the fact that you so thoroughly enjoy playing the instrument you love and everyone else ignores is enough for you. You don't dominate the stage, nor do you even wish to anymore. Those days are long over. You have your audience, but you'd be satisfied even if it only included yourself.
That's why all the good bass players always stand there on stage with goofy smiles, their heads bobbing around, and generally looking as if they're having the time of their lives.