Wall cabinets aren't flat like paintings (or at least they don't look that way), so you should be able to place items on top of them like with large cabinets.
+1
i want to place things on top of SDB's too! I can put normal items on them, but not decorations, and cluttering up every available surface is something I love about decorating.
I also like to be able to drop something from my inventory into my cabinet without opening the door*... I can imagine that you probably wouldn't want both of those options at once.