Don
"the same performers will show up at most, if not all, of the session venues "

That's not my experience and if they do it's usually to grab another moment of glory. Increasingly, the trend also seems to be that "booked" musicians have become more aloof and/or prefer to start their own sessions.

Howard,

"Some might equally complain that every session is less opportunity for a club-style event where people can simply sit and listen without feeling pressured to perform themselves"

They aren't obliged to perform in a session or singaround either. Most of these are held in pubs where other customers are welcome to listen or not.

There's actually more pressure if it's set up as an "open Mic" affair or having to perform in front of an audience and everything is much more controlled.