I have been using hammocks for about 4 years now, maybe 5. I used a bag and was very happy with it until my wife bought me a top quilt as a gift a couple of months ago. My TQ has loops on it, for connecting it to a mat if I have to go to the ground. It is so much better in the hammock I can not really explain why, it just is. Now that I said that I still would use a bag and still will use one in the summer.
My reason for the TQ was for winter camping when you have to carry more in your pack a TQ, for me saves on weight and space. I dropped 2 pounds going to a TQ for winter.
Guess what I am trying to say is if you have a bag use it. You will be fine. If you can get a TQ you will like it and not regret the purchase.