Now, let's say that I have a 3 phase mill. Power goes out. What are
the possible consequences of me forgetting to turn it (and RPC) off?

The answer is, the same as in the single phase example above.

You are correct. I believe this is acceptable for a residence
but I SUSPECT its not allowed for industrial applications and that
is why its commonly used.

There are some applications were auto restart is not allowed and
this is why they make make single phase motors with manual reset
overloads too. I have noticed lots of warnings in Grainger
catalog about auto reset thermal overloads on motors. They want
to make sure that the person working on the machine isn't surprised
and injured when it restarts by itself.

Some VFD will also not allow auto restart. My Boston fincor VFD will
not start the motor when power is applied with the switch in the
RUN position. In this case the switch has to be moved to stop and
the reset button pushed. The Teco drives are not as smart. They
attempt to do this but will auto start if the power is gone long
enough for the VFD to forget its previous state. I am still somewhat
dissapointed in this. All my VFDs will flash overload and
wait for manual reset when the motor is overloaded. This is done
to emulate the 3 wire control and autodrop out of the contactor.

Most people here tend to treat their shops in a conservative manner
and implement some of the industrial safety measures for their
own safetly. A good friend of mine lost a finger when a machine
started on its own. I want to keep mine.

chuck