I carry the opinion that the purpose of a jam band is to take the music someplace new every time you play. You can't really do that if you just play the same pattern for twenty minutes while the guitarist solos over it. Change it up, and change it up frequently. The notes and the groove. If you have a good drummer you can connect with, hopefully he'll change up what he's playing and you can follow him.
For me, the epitome of the jam band is Cream. On their albums they keep the songs to a normal length, but in their live shows they stretch them out for 10 - 20 minutes, and the songs are different each time. Jack Bruce doesn't just play the Sunshine of Your Love riff for 20 minutes. The rhythm section is constantly changing what they're doing.
I like this example, despite the terrible quality: http://youtu.be/Wl1Scq4LIEE
They start into the vamp at about 2:45. It takes Jack and Ginger about 30 seconds or so to lock into each other, but once they do they never break away. Jack follows Ginger through all his fills and changes. When they alter the beat at the 4:00 mark, the music is so tight it's like they're reading each others minds.
I agree with the general sentiment that you should play what the song needs, but the song should never need the bassist to play the same bar of music over and over for an hour.
Truth +1