Two things about camping on the summit:
1. You have to carry enough water to make the climb, spend the night, and then some to get back down. Each time I have done it, I've started out with 5 or 6 liters of water. (I have used Zip Lock bags, quarts bagged inside gallon bags. They work ok, but can leak a little at the zipper seam ends.)
2. Altitude: Unless you have spent at least two days at significant altitude, a night on the summit can bring on some AMS. One time, I had to get down before I could eat any breakfast.
Lots of JMT finishers like to come up from Guitar Lake to watch the sunrise, so there is often a group arriving just before sunrise. One time, I was in the hut, and a hiker made some Starbucks instant and shared with me.
