A polarity reversing switch will instantly change the direction of the
motor, regardless of whether it is in motion or not. This is
incredibly dangerous. Think of how much strain you put in the motor
spinning at XXXX rpms and you instantaneously make it turn the other
direction. The jolt to the setup would be unnerving.

Now think of having something chucked up, and you switch directions at
the flip of a switch. The inertia that a spinning piece of wood might
have spinning at a modest rate (say 2000 rpms) that is in effect being
slammed into a dead stop (and at the same instant - reversed!) would
throw just about anything off the chuck/faceplate/centers. And with a
10 - 20 pound piece of wood on the spindle, how much force would be
retained by the launched wood, and how much would it take to bend the
lathe spindle shaft or break your tailstock?

There is a guy in our turning club that has a motor assembly he bought
somewhere that has a variable speed reverse. He said that when he
doesn't remember to stop 100%, it sounds like a pistol shot when he
just hits reverse. I can't how that is a good thing...

Robert