Thanks for the comments. An interesting mix of open mic versions. And yes, if the session goes on for many hours, I wouldn't expect everyone to sit through everything. But would an couple of hours be too much to ask?

Apparently so for a few.

On further contemplation of the local offenders, they seem to fall mainly into 2 camps.
1. Those who haven't been there much (if at all) and don't really feel part of the community yet, or haven't yet got into good habits.

2. One of two old hands, who feel they have 'paid their dues' so don't need to show any respect or support for newer musicians.

As this had come up in discussion with a few friends, I even penned a blog post about it.
Blog here