warning: this is coming from a highschool blues/jazz bassist, and I know a lot of people don't respect young musicians.
I was playing today with my rythm section during my jazz band period. I am in the jazz A band, and we also have a a jazz B band for beginners and extra practice. Well today the B band bassist came into our little jam, and he said he wanted to jam. I gave him my bass, and we were off.
Soon after the first chorus of "The Wind Cries Mary" (oh btw we had the day off today, we are usually working our tails off on music, we got a break today

) the kid was lost. He asked me for tabs and notation, and all I knew was the chord progressions, and the basic rythms. When I jam, I am always making up lines off the top of my head, and simply following the rest of the band until I have a little confidence to become a bit fancier.
This kid couldn't follow anything, and as a result couldn't hold a solid groove. He can read quite well, but when asked to lay it down thick on the spot, it became a problem.
I know that some people need to see things to understand them, but it is also a necessity to have a good ear. You never know if your guitarist is going to extend a solo, or if the progression will change, or if you need to improvise. This can only be acquired by playing more with your ear, and less with theory all the time (although theory is EXTREMELY important)
I guess my point is that if you like to jam and play jazz/the blues, always keep an open mind and an open ear. I plan on working with the bassist from the B band for a while now so that he can take his head out of the music and open his ears to the groove arond him! It should be a fun, yet challenging experience!
just my .02$
Joe