Well, I suppose there is that. The other reason is (was) that I'm using a PSX controller hack, and I've run out of buttons thus:
2 x joysticks (hacked from the analogue stick ports to be made digital)
6 x player buttons x 2 players
1p and 2p coin
1p and 2p start
However, something I completely didn't think about is that I can just duplicate a couple of buttons by wiring up the pinball buttons in parallel to two of the main CP buttons. Afterall, I am not going to be using the pinball buttons in Mame, nor the main CP buttons in Visual Pinball.
So I guess I've solved that one and can just find somewhere else to wire up the mouse buttons (seems a shame to waste them by not hooking them up!)