I've read entirely too many negative threads on behavior at shelters, so I just wanted to post a positive experience for once.
On my section hike last week, I did a "short" 12 mile day NOBO from Bland, VA to Jenny Knob shelter. I'd originally planned a "long" 18 mile day to Trent's Grocery but decided to cut it short after seeing an 80% chance of afternoon thunderstorms in the forecast. I arrived at the shelter just after 2:00 and there was only one other person there. I'd been out for a week and hadn't seen many people on the trail so I set up in the shelter figuring at most 4 people in for the night.
Around 3:00, a group of 4 NOBO weekenders came in. A little surprising since these were the first NOBOs I'd run into in a week, but it was the weekend after all. I left my stuff in the shelter and hoped no one else showed up. Then around 4:00, two SOBO thru-hikers showed up. They told us that at least 6 more were coming. At this point, the sky was getting darker and the wind was starting to pick up. Much to my surprise, everyone agreed to set up their tents and save the shelter for people who were coming in later.
So we all set up our tents, cooked our dinner, and sure enough a total of 8 more SOBO thru-hikers showed up between 5:30 and 6:00 while we were eating. (Apparently, they'd all gotten bunched up in town waiting out 10 straight days of rain.) By this time the sky was dark, the wind was blowing pretty hard, and thunder was rumbling, but the shelter was empty and there was just enough space for all the newcomers.
At 6:15, the sky fell out and it rained pretty hard for several hours. There were a total of 16 people at a shelter designed for 8, but everyone stayed dry and there was no complaining or fighting over who got to stay in the shelter.