What does a rhythm guitarist do?

[quickfix]

The rhythm guitarist is the basis of the band. Everyone else plays the licks around him based on what he's playing.
Take him out of the equation and everyone else just sounds like they're waxxxxx xff.

The only musician that can replace an accomplished rhythm guitarist is an accomplished piano player.
Either of these two musicians play a complete instrument that manages to simultaneously play harmonically sophisticated chords, and hold down a driving beat.

A singer can sing a complete song with just a rhythm guitarist or pianist as accompaniment. Not so with the other single note instruments in the vast majority of cases.

 

[Luke19Boarder]

In my experience playing in bands, the lead singer usually plays rhythm guitar (not always the case, obviously). They focus primarily on singing, but add less-intrusive guitar as well.