Does a variable speed drive need STO?

From an amplifier design standpoint, the Safe Torque Off (STO) is redundant with the enable input to the amplifier. I'm not sure what "problem" this STO requirement is trying to solve that the enable/disable doesn't solve, unless there's some variable speed drives out there that don't implement the enable/disable correctly and tend to take off when not commanded. In our amplifiers, in addition to stopping the PWM outputs from toggling during a disable condition, the DSP also clamps the output stage off through a secondary circuit by preventing the PWM signals from going into the output drivers. The clamp circuit is a failsafe circuit meaning that if the DSP is not under control, the default state of the clamp is off. I don't really see a need to have an additional external STO circuit to do this other than the fact it allows someone not familiar with the electronics (or doesn't trust the electronics) to be assured the output stage won't send torque to the motor.

If that's the case, then from an outside view, I would much rather be assured the bus power is removed from the amplifier, not just that the output stage can't move the motor (while bus power is still applied). Bus power can still go to the motor under the current STO requirements.