If you listen to a band whose bassist is absent, you'll realize the importance of bass to the rhythm section. Bass provides an essential bridge between the drums and the melody. Most people don't realize how important it is until it's gone, and even then they can't quite put a finger on what's missing. They just know something is horribly wrong.
Bass isn't a glamour instrument, but there are lots of musicians who aren't making music to call attention to themselves. Orchestral players, for instance, are passionate about being part of the overall sound. Each of them is vital to the music, even though it's unlikely any of them will be singled out for adulation. It's much like that with bassists. Being part of the rhythm section, a bassist locks into a groove with the drummer and certainly isn't limited to four to eight notes. He/she supports the drummer and adds depth to the song that can't be achieved by trebly guitars.
A lot of guitarists assume they can play bass because they treat bass as though it were a four-stringed guitar. But the term "bass guitar" is entirely false, since the bass isn't a guitar at all. The only similarity is that electric basses are visually similar. But they're just portable, more easily amplified versions of the stand-up basses that have been a staple of music for a century. When people played stand-up basses, nobody regarded them as failed guitarists because audiences realized bass was an instrument in its own right. But since the advent of the electric instrument age, people have lost that understanding and confused bass and guitar. Most guitarists don't even know the difference, which is why guitarists tend to make horrible bassists. Since they're accustomed to playing guitar, they overplay -- cramming as many notes as possible into every song. They have no idea that doing so just makes everything sound muddy and chaotic, since they're so accustomed to showing off that they've never familiarized themselves with the role of the rhythm section. In contrast, bassists who switch to guitar tend to make great guitarists because they don't try to minimize the significance of the rhythm section.
Another advantage to being a bassist is that you can always find a band in need of your services. Way too many people approach music with the sole motive of being the center of attention. As a result, there are billions of great guitarists competing for a comparably small number of jobs. Every time I've run an ad seeking a guitarist, I've gotten hundreds of calls from mostly arrogant people who invite me to ride their coattails to fame and fortune. But when one of my bands runs an ad for a bassist, we're lucky if we receive a half dozen responses, since bassists are few and far between and very much in demand. A mediocre bassist can find work with a fantastic band, but a brilliant guitarist has to compete for work with hundreds of other great guitarists, most of whom think they're God's gift to music.