In both of my bands, the guitar players write good music (i.e. The kind of stuff that I think makes good music).
In one, the guitar player prefers to play rhythm parts. He never plays too loud and trys to make sure that everything is eq'd. More often that not, he'll ask after the first couple of songs if people want to hear more bass, regardless of how loud I actually am. His belief about my bass parts is that they should serve as a melody as much as the voice or guitar parts should. He also doesn't mind it (Makes extra space in the set list for it too) if I want to play some solo bass stuff during our set at any point (I usually keep that for times when I know that there are lots of musicians and friends in the audience and, even then, I only play one song unless they ask for more.)
In the other, The guitar player plays a bit more in the way of melodic lines in addition to rhythm. If there is a spot for a solo, we discuss whether the drums, bass, keyboard or guitar gets it, and he usually doesn't take it. When I asked him why, he said that "Every song already has enough guitar in it. It would sound like too much if I played too many guitar solos. Some of the songs already sound like guitar solos anyways so I'm happy."
I love my bands, although, I would eventually like to lead a band myself.