These motors can have extremely high acceleration due to reduced weight of the rotor. The rotor armatures also have low inductance. This helps in extending the life of the brushes and the commutator.
These rotors have low inertia and low noise and vibration and thus can be easily controlled.
The downside to these motors is that the rotor gets heated quickly as it does not have the core which can serve as a heat sink. These motors can be used only for low power application such as the motor in magnetic tape devices, optical rotary encoders, etc..
image courtesy: www.micromo.com