[jacquesm]

> accuracy and repeatability

And reliability. Making driver electronics that will live long term is not that simple, sudden stops and overload conditions are tricky to deal with in an effective way without risk to the attached electronics.

> They know a step was performed. If not, you can act on it.

Servo's don't 'step', current is applied to the motor in a continuously varying manner and the delta between the desired movement and the observed movement determines how that current will change over time.

This leads to all kinds of nasty side effects: overshoot, undershoot, runaway in case of a failing feedback mechanism and so on. Steppers are much simpler to interface to in principle (but to drive them at speed is remarkably hard due to all kinds of resonance issues) but much harder to get performance and reliability out of, almost all larger scale industrial control is done with servo motors tightly coupled to their driver electronics.