We-e-e-ll... you can have a scenario where people "take turns" to lead off on something, and everyone joins in - and that is certainly one form of a session. Semantics don't matter - it's what actually happens that matters.

What's the inherent value in an evening's music being unstructured? I would have thought that giving everyone of whatever standard the chance to lead off on something would (a) encourage the lesser-skilled players (b) prevent others hogging the choice of tunes.