I hate wires coming out of my mice. I can tolerate the thin braided cables coming out of most high-end mice, but that's about it. With Mouse Box, I'd have display, USB, and a bunch of other stuff sticking out of the mouse making it much harder to move around and use.
The mouse itself looks unexceptional. It's not especially ergonomic (my Logitech MX Revolution looks much more comfortable to hold), and the button set is pretty limited. It's fine for basic use, but it would kind of defeat the point if I felt compelled to plug a mouse into my Mouse Box.
Mice wear out. Mine collect dust and scuffs on the bottom and sweat and dirt on the top. I guess the mouse itself could conceivably outlive the usefulness of the computing part, but it seems like a bad idea to put the computer in a consumable device.
The use cases seem a bit contrived. I might accept the projector one because clicking to advance through slides is something that people do, but surreptitiously using it at work by connecting it as a second input to the monitor sounds pretty far-fetched.