I usually drink beer at gigs if it's free. My cover band played either three or four sets a night so I'd drink three or four beers over as many hours. In my opinion, that is reasonable and responsible consumption. My bandmates were about the same.
In my younger days (my early 20s, new to the gig scene) we'd bring a cooler with us. 8)
I found the problem wasn't getting drunk but rather having to visit the men's room rather urgently when you're only 15 minutes into a 45- or 60-minute set.
I would gladly play a gig without alcohol and have done so, but I do like beer and when I'm in a bar playing rock music surrounded by beer drinking customers and bandmates, I'm gonna bend my elbow too, 'specially since they have my favorite beer: FREE!
(BTW, tip the bartender!)