[slomobile]

Reading 2015 R42 Table 4-4, it is legal to power electrical solenoids with the PCM. What is a stepper but 4 solenoids arranged about a steel rotor?
If each coil of a stepper were connected to a solenoid channel, and the channels were activated and deactivated in the right sequence, the stepper would function correctly. It would not be fast or efficient code, but it would work, it would be safe, and as far as I can see, it would be legal. Getting a robot inspector to agree is a completely different matter.

The inherent current limitation of the PCM ensures that unsafe forces will not be developed.

Useful applications of steppers in FRC include camera turrets, gating, and autonomous aiming mechanisms where a device needs to be rotated to multiple positions with a high degree of repeatability. There are some applications where there just isn't room for multiple limit switches or the degree of movement required is so small that it simply cannot be achieved by brushed motors without overshoot.
All the warnings about becoming out of step mentioned earlier in the thread still apply, but a home limit switch can mitigate the effects.