What a 3d mouse would offer to Inkscape is the ability to draw and move at the same time. Once you get used to using one, you feel as though you are interacting with the object in the real space around you (3d object in a normal 3d modeling program, but in our case, the canvas), much like you might move your head while drawing with real paper and pencil.
This video gives you some idea of how it would be used in Inkscape (it's being used in Photoshop in the video):
What would be desired in the use of the 3d mouse would be smooth zooming, and of course standard panning. The panning would be much like panning with the space bar held down, except that you don't need to reset the mouse position after you've panned one screen width. The zooming however, IIRC, is stepped, and would be too jerky. I don't know if this is simply how the mouse wheel is setup, implying that the underlying rendering engine could support "smooth zooming", or if it is in fact a limitation of the renderer.
If a developer is interested in working on this, I'd be happy to send them a 3d mouse for free. FREE STUFF!!!!
-John