They not only provide the interface between melody and rhythm, they invariably have to manage the insane and obnoxious egos of the band to glue the whole thing together. And it's not which notes you play and how many strings there are, it's when you play them. Most guitarists are ****ing shít rhythmically.
Also it has been scientifically proven that the bass is the driving pulse of rock music that make people like it. So ner.