Been there myself a few times!
It's not so bad when you know beforehand that you're there just to provide "background". The whole approach is different. No need to "connect" with the audience, just get on with getting paid to play and no feeling of dissapointment at the lack of applause etc.
100 times worse when you're under the illusion that the audience wants to be entertained, and you pull out all the stops until it dawns on you that you wouldn't draw a glance if you set fire to yourself!!
Even knowing what to expect, I hate doing those "wallpaper" gigs. I'd rather play for nothing to an audience that wants to listen.