The Opti-Wiz elegantly avoids this problem, but the Z-axis setup is fairly MAME-limited. (In other words, if you wanted to use your spinner with a PC driving game, or most other non-MAME emulators, I don't think most of them would allow you to control the action using (for all intents and purposes) the mouse scroll wheel).
Is this the case even though Randy has explained that his chip far exceeds the performance of regular mice? I obviously know none of the technical details behind the Opti-Wiz, but Randy's explanation leads me to believe his board my be able to use all 3 axes at a much higher refresh rate/speed.
If this is the case, and the Z-axis is actually fairly robust in terms of speed (say, in line with the X and Y axes of a regular mouse), it may be possible to use the spinner in non-MAME games.
Just a thought. Hopefully Randy can let us know which is the case. (Or some of the beta testers)
EDIT: I had a bit of a brain fart there and totally missed the main point of your post Tiger.

I guess it wouldn't be very common for newer PC games to allow you to map controls to the scroll wheel. I'm still curious about the speed of the Z-axis on the Opti-Wiz compared to a run-of-the-mill mouse though.