These mastubatory sessions or jsms or whatever you may call them serve a couple of real and valuable functions: they give instrumentalists a chance to practice; they help instrumentalists learn new (to them) tunes, and they're fun to participate in.
One caution, though: they shouldn't be listened to, unless you're fond of hearing a dozen or so soloists trying to play in unison (albeit not always in tune.) It's a crying shame that ensemble playing for non-performance and non rehearsal purposes has virtually vanished.

The problem lies, not with old-time music, but "old-time" musicians.