I did my first fleece as backing only a couple years ago. I do prefer a bit of something between the layers, even a piece of flannel to help bite/hold the stitches but I don't think I want a thick batt unless it was going to a very cold home. Also, some fleece can be very dark/heavily printed and show through a light top if there is no interfacing of any type.
It was simple and easy to work with, no problems at all. I spray baste. I think it is a great option, especially when it goes on sale. The one thing that comes up is keeping the project to the width of the fleece or the options for seaming. But for a single width project, it's great.
The one down side is the same reason I don't back with flannel, I have pets and don't like the way the hair grabs to it.