Agreed! When my son graduated from the L.A. Music Academy 15 years ago, I took him to several jams in the greater L.A. area. over the course of a year or so. Some were great (Monday night jams at The Baked Potato were fantastic and the norm - others were cool as well.)
There was one close to home for us which was hit or miss, as the "host" band would rotate. One night was especially "memorable," as it was an older band. My son had joined the jam with the intention of playing on 2-3 songs, but the host bassist went MIA and left him up there for an hour playing with rotating jammers who couldn't play and couldn't sing.
Memorable, but not a good experience.