I go to open jams about as often as I can, more now that I live in a new city. For me, they have a couple really great benefits-
1- You meet other musicians, get to hang and talk shop, and have the opperunity to pick up gigs and invitations to audition for projects. If I'm starting a band or looking for a sub, I'll almost always think to someone I have jammed/hung with in the past and liked before I ask other musicians for numbers or hit CL. Some people are hard and unfriendly at first, but if you have a good attitude and are there to learn, I've found even the kings of the scene will come around to you after some time.
1a- You don't need to play all night or hang all night. There probably will be other bassists who will be happy to trade off, so you can grab a beer/soda and not worry about treating it like a gig where you have a million responsibilities.
2- While you shouldn't just play whatever, jams can be a bit of a sandbox for learning new things. From a bassists perspective, it's a great trial-by-fire to pick up key signatures, progressions, and arrangements by ear. Sure, it takes practice, but open jams allows you to develop these skills in an environment where everyone attending is there to get better, as opposed to gigs/rehearsals where everyone is there to do a seamless performance or prepare for one.
2a- Learning songs on the fly at jams helped me break out of the "I need to have everything I plan on playing memorized before I walk in the door" crutch and develop my ears/musical vocabulary, and nothing has made me more employable as a professional bassist than that.
3- You learn other people's tunes, and tunes you know transposed to different keys, which prepares you to take new gigs and break into new call list. You can also teach other people the songs you play, but learn the words/melody if you want to lead tunes. Some jams will let you bring charts, others look down on this.
4- When I go to jams, I never fail to both impress and underwhelm over the course of the night. Stuff that sounds boring in the shed sounds really funky in the mix, and things I think sound really hip in the shed fall flat. It's a very honest wake-up call for what I'm doing right, and what needs to be improved on the play the music I want to play, with the people I want to play it with.
OP, I get you are a little apprehensive but I think it's worth a try. You honestly have a lot to gain and little to lose.