Good question. A few reasons:

1. I like to drab out my N800, because I am sometimes paranoid about people on metros and other places thinking I'd be a good person to rob. The grey case I now have makes the N800 quite nondescript.

2. The case gives a certain measure of protection against spills or scratches. In my pocket, for example, the area that could be scratched by car keys is reduced a lot.

3. Most importantly, a little rubberization adds a tremendous amount of protection against being dropped because it makes the N800 slightly bouncy.

4. The N800 has a tendency to slide. If you set an N800 on a slippery desk or on the metallic surface inside or outside a car, it only takes a slight nudge or bounce to send it off an edge. If you have the N800 covered with something rubbery, it tends to stay where you put it.