I've been (successfully) cutting threads for years, but a quick retracting toolpost would greatly improve my metric threading. I do things a little unconventionally - Make sure you read to 'You have to keep the half-nuts engaged for metric threading - not! . And this seems like a real time saver.I don't know what it is about threading that makes people think that they have to make multiple moves faster than the speed of light at the end of the thread they are cutting. All you have to do is disengage the half nuts for imperial threads or stop the motor on metric threads. The tool bit can happily sit there cutting a groove. The groove won't have any affect on the thread. Then you can calmly do what you need to to make another pass.
If all you did was cut threads all day long and having this gizmo would let you cut more threads in a day then by all means get one. Or if you want a cool project to make then make one. Otherwise why spend the money on something that you really don't need.
GsT