Originally Posted by mr dave

It's more like a situation where Jam Bands seem to plan to 'not have a plan' during particular spots in their songs / set.

The other thing I notice is Jam Bands tend to stick to simple and recognizable patterns. Lots of I-IV-V progressions, nothing out of the ordinary, if there's a D and a C during a part then you can pretty well bet the other chord will be a G.

The whole point of Prog bands seems to be to eschew those traditional progressions to create something new and unique, which kind of requires organization, even if one musician is expected to improvise over a particular element of the composition.