At this point the question is why the iMac still comes with the mouse. The Magic Trackpad is much better and all of Apple's software is designed for it.
By default, Apple expects you to own the Magic Mouse. Which is probably a part of their plan to upsell you to the Magic Trackpad - the rift in quality between the two accessories is so great that it's hard to imagine they're made by the same company.
I am wrong, but I think my point still stands. Very few people ever use the mouse. It would probably be better ROI for them to refine the design of dongles or something.