Yes, typical fleece is pretty much useless against the elements, but the new laminated fleeces are arguably an improvement over wool.

Unless you get a sleeping bag with a weatherproof shell, it is about worthless out in the open. Get one wet and see how comfortable it is to snuggle into when ambient temps are low.

I can take my laminated fleece blanket and lay on it on wet ground, wrap it around myself, and I am protected from the wet ground and from the elements above. Water won't seep through the material, and wind won't penetrate, except where it is open. The nap won't compress, so the cold spots are minimal.

Wool is a good alternative, and a decent sleeping bag is excellent when used within it's limitations. Laminated fleece is going to outperform both wool and any sleeping bag under the worst of conditions, unless you augment the wool or the sleeping bag with something more. I carry just one thing, the laminated fleece blanket, and it does what I need it to do. Sewing a nylon backing onto a standard fleece blanket is not going to be anywhere near as effective either.

Anything is better than nothing I suppose. If you want the maximum protection and durability available, you will go with laminated fleece.