I've used fleece and found it very easy to work with, but I don't like the lack of squishiness of no batting but then I am typically sewing for a cool climate. For a functional blanket that can be used as a blanky or used to build forts and put down on the ground for a picnic, it can be great.
I do prefer something more than just the top and fleece and for dark colors showing through it is pretty necessary. Muslin or flannel (pre-washed/shrunk) would work fine.