The mighty mouse does not support clicking of more than one button at the same time. I gave mine up for this reason. The whole thing seemed form over function. I am back to my S+arck mouse, which is a bit random on the optical side (I have two and they are both the same) but it has great buttons/wheel.
Scrolling should be for Zoom (as it really only works in two axies). Pan is great with mouse movements.
So it makes sense to use the wheel for zoom and the wheel click for pan.