It's a computer mouse. It's bound to pick up scratches with heavy use. The scratches will show up in certain lighting conditions more than others just like scratches on anything (not just computer mice).
If scratches on mice really bother you, buy five $20 cheapos and discard yearly. You probably won't get the Magic Mouse-specific Gestures, but then maybe you don't care about such matters.
My guess is that most scratches actually come when the mouse isn't being used, abrasion during transport. If you want to preserve the aesthetics as much as possible, stick your mouse in a cloth bag before you shove it into your backpack, etc.
That has worked really well for my Magic Mouse 1. The plastic rails on the bottom exhibit wear naturally but the rest of the device is pristine.