What's laughable is that often people whining about "jammers" can't compose their way out of a sopped liquor store bag. While some of the improvisers have a high individual and/or collective sense of composition.
My experiences say the exact opposite. Jammers are people who can't write to save their lives and often fall into playing the same mundane licks over and over again because they work. Composers actually sit down and work out new things because they want to expand from the norm.
I've played in about 4 different jam bands within the past year (around 20 of so different players) and while it would be cool for the first rehearsal or two with a group, I'd very quickly realize that all the players would play the exact same licks and progressions over and over again because it was all they knew how to do... and then they'd suggest Phish song after Phish song to cover...
Sorry, but jammers are the biggest waste of space on the music circuit, and they unfortunately make up the bulk of musicians in the college scene.
Jamming =/= Improvisation