My best friend used a Wacom that came with a "mouse" which was essentially a plastic rock with a tablet pen node on the bottom. I have had to use it multiple times when I forget to bring my mouse over. It is terrible and should never be used in place of a mouse. The tracking is imprecise, the shape is uncomfortable, and the roaming space is limited by how large your tablet is.
Let me clarify something about roaming space. Some of you may use high DPI mice and don't really need much "roaming space." But that's not the weakness here. The weakness is in a desk that changes. Wherever you want to MOVE the mouse (not necessarily move as in using it but also as in relocating it), you have to move the tablet with it. It might not SEEM like a hassle talking about it, but if you have papers, drinks, folders, pens, etc. on your desk moving around a mouse attached to a permanent mouse pad is really annoying.