If you are trying to get there and need some exposer, then I see no reason why you shouldn't take the free gig. It is sometimes better than not working at all, plus after the gig, if the guys really feel like you did a great job, they all might pay you. Don't make a big deal out of it and don't play too many more of these.
like you said, you are networking. Sometimes the way you handle yourself, your equipment, their equipment and the situation says a lot about who you are.
I would show up early, help them set up as quickly as possible and help tear down as quickly as possible when finished. I would act as if this were my own band and be respectful of all the band members... If you get paid, great if not you at least make a very good impression and other bands might talk about you and the networking begins.... "Yeah - Tom is an awesome bass player, he really knows what he is doing. He even helped us set up and tear down.... He was just subbing for us, we really appreciate him" When your name gets around and there are great stories to tell about you, then you get what you really want.... to get paid.