M.P.
I AM a full day kindergarten teacher, and there are many benefits in it. It's not just "filler" activities that go on throughout the day. You are WAY less rushed to fit everything in with full-day, and the extended day allows the teacher and students to stay on a topics if the little ones aren't really getting it. The teacher and students can get WAY more in depth with different topics than you can in half day. this supplies the little learners with different ways to learn and solidify their knowledge.
As for the melt downs, some kids have those at first (not as many as you would think), but I can count on one hand the amount of times a student had one after October. Plus, the kids who are in full-day kindergarten are way more ready for the first grade schedule than those in half-day. A good teacher knows that they can only keep little minds on-task for so long, and the activity gets switched up fairly often.
HTH!