Depends upon your personal tolerance for cool water. I used a 3mm full wetsuit the first day in Cozumel, and switched to the 1/2mm full suit for all of the other days (two trips, 22 dives) I have never had a problem with getting cold, but some people get chilled even with a 3mm full suit. YMMV.
There are things in the water that you do not want to come in contact with, so a full length suit is considered a good idea by some people. Yes, we all know that you are supposed to be able to stay off the coral etc, but sometimes stuff happens.
I also wear the full suit because it provides me some extra degree of protection from the sun. Yes, it can get warm sitting in the sun, but a wetsuit makes very effective sunblock!
Wristshot.