My car does have heated...well everything, but I've never seen the benefit of the heated mirrors. They've never fogged up on me.
In the snow belt the wing mirrors can frost up at night and become covered in snow when driving in a storm. The vortex off the mirror swirls the snow around until it sticks to the glass. Heated mirrors keep the snow melted so you can see.
On most cars the heated mirrors turn on when you turn on the rear defroster.
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