It's going to be very dependant on your machine. If you have a low powered spindle, it seems it will stall before snapping. Presumably on such a machine, the axis drives are likely to bog down if the spindle stalls, in which case(s) you appear to be able to kill the drives in time to save the cutter. It wouldn't be much use to me, based on my own unfortunate experiences, as it's the axis motion that causes breakages pretty much instantaneously when the cutter stops cutting - usually due to recutting / chip welding in my case. The answer to that is chip evacuation by air and/or coolant.

It would also be redundant if your VFD had a "spindle at speed" output but again, it's only going to be relevant in some applications. Although the Yaskawa spindle VFDs I've used on my machines have that feature, it would be of little use to prevent cutter breakages. Instead, its used to allow the spindle rpm to reach the programmed speed before the next G or M code is executed.