Useful, yes. Essential, not really.

If you have a heated front screen, on a frosty morning you can start the
car, switch both screen heaters on, then get out to spray the side windows.
By the time you get back in, the windscreen has started to clear. By the
time the screen has fully cleared, the mirrors almost clear.

You should not scrape the mirrors; they are surface-silvered, and very easy
to scratch.

In the UK, at this time of year, mist can form in low lying areas even
though it might be clear where you live. Driving through these misty
patches often fogs the mirrors; this is where the heated ones are most
useful.

Chris

Its circuit is linked in with the quick clear screen... but I find it more
useful for those autumn and winter mornings when dew is on the mirrors...a
minute with the quick clear screen on clears the mirrors which I find
essential to reverse out of my drive in a morning.

Useful when wet weather + rain change to below minus 1-5c overnight.
Heated the ice melts from the mirror surface, and one can clear the ice
piece away by hand.
The above will probably never happen to Mr. Bond if in UK. :)
/Jörgen