The mouse still works, right?
Do you return your car to the dealer when you get scuff marks in the carpet or elsewhere in the interior? Or if you get a scratch on your iPhone, etc?
My PC mouse here shows shinyness in the plasic where my hand rests (its worn).
My PC laptop's trackpad shows similar, as does every PC keyboard I've ever used (after say, 3 months).
As the apple store guy said, they're not intended or engineered to remain looking perfect forever. If the thing still works, it does the job it was sold to do and thus keeping up its end of the deal, in my opinion.
Things (in general) will show wear and tear if used. Apple hardware is no different.