My only experience of jam nights is being the house bass player for a year, finished about a year ago. I'd never been even been to one before.
Have to say I don't really agree with some of the things I've read.
We had a couple of nights were basically we played for pretty much the whole night as there wasn't anyone other players there but all in all I considered it a failure if the house band we're playing at all other than backing someone. We were being paid and as far as I was concerned it wasn't about us. It was about other people getting chance to play, using us as they saw fit or doing it themselves. Quality was never an issue, it's a jam night and even if you were rubbish but you wanted a go that was totally the point.
Had a couple of busy occasions were I was apologising to people walking out at the end with a guitar case that hadn't been opened cos; it was too busy and that felt like a fail too.
There was nothing cliquey about our jam night and upyourownarseness was certainly frowned on.