Aside from the technical issues of lack of equipment use, for someone who only gigs occasionally but wants to be ready for that gig, I think it's really good to practice on your PA at least once a week. Going to play open mics, you don't want to get there and have to adjust to the different way in which the guitar will respond plugged in. If you're singing, you also want to be accustomed to how the mic will respond differently than singing sans a PA, so that when you get to the open mic you're accustomed to mic'd singing.
That's just my take. As a sports coach, I always say, you're going to play the game/match like you practice, so it's good to replicate the situation in which you're going to play live as close as you can so that when you get there, you're ready, and the situation doesn't seem foreign.