A cohesive, good sounding bass line over a chord progression is a f**kload harder to improvise than a wanking pentatonic solo over the same progression. It's so easy to solo over F, Em, Dm, C, but as a bass player you would have to think something like F, A, C, E, D, B, G, E, D, F, A, C, B, G, E, C. Now the reason this is harder is that you are responsible for outlining the chords - the notes matter (that, by the way was ascending and descending diatonic 7th chords). As a guitar player, you can pick any note you want. Bassist aren't tools unless they're in Blink 182 or some other dumbass band. Good bassists are integral to good music (picture Dream Theater or Tool with a bassist only hitting the root notes...)
Ever heard of Victor Wooten? Stu Hamm? Bill "The Buddha" Dickens? John Myuang?
"You must stab him in the heart with the Bone Saber of Zumacalis... well, you could stab him in the head or the lungs, too... and the saber, it probably doesn't have to be bone, just anything sharp lying around the house... you could poke him with a pillow and kill him."
- Aqua Teen Hunger Force, The Universal Re-Monster
