I train with the Gi and unlike no Gi you are forced to learn the technical aspects of BJJ. We have a guy in our gym right now that has trained No Gi and is now with us and he was completely losted the first few nights. With the Gi on, not only are your more resticted in your movements, but it is hot as hell sometimes! You learn not just the move, but how to do it correctly.
If you are only training for MMA, no Gi is understandable, but I don't know anyone outside of the ring that fights in tights (that would be a little weird). Trainig with the Gi shows you how to use what the person is wearing to submit them or to advance your position.