Variable Speed Drives today can be run with or without sensors. By sensors, we mean encoders, tachometers or resolvers. These are attached to the Opposite Drive End of the motor and give an RPM feedback of the motor (not roller).
The encoder costs several hundreds to $1000 plus wiring. If the drive will run without an encoder, why use one?
You will need an encoder on and unwind or winder for roll diameter calculation. Also, the drive needs the best possible performance for unwinds and winders.
If the line is threaded using the drive at low speed, I would recommend encoders.
If the roller must stop or start rapidly, I would recommend using an encoder.
If your product scratches easily and speed must be spot on, I recommend encoders.
Most rollers in the middle of the line do not required encoders for good performance.
That would include calenders, cast rollers, slitting sections, nips, vacuum rolls.
Sometimes drives do not tune well without an encoder. Then you will wish you had one. For example, an off-loading cart may not run well at low speed. Threading conveyors may not hold speed well enough. Old motors may not auto-tune with new drives. In cases like this, I recommend an encoder. You can probably benefit from just one encoder and tune other similar encoderless drives to match. The encoder provides a measure of assurance that all is correct.