In considering how likely is a specific motor to experience rotor bar defects, we prioritize our frequency for current signature analysis (which we use primarily for broken rotor bar monitoring, even though others use it for much more) considering the following factors:
* frequency of motor starts (frequent starting is the worst)
* inertia of the load (high inertia is the worst - primarily fan motors or loads with flywheel such as RCP motors and rod drive MG motors)
* motor speed (2-pole is worst)
* horsepower (high horsepower is the worst)
* on-site failure history (we have only had rotor bar failures on one family of large motors so far).
In addition to inertia, other factors which affect the severity of a given start are voltage (low voltage is worse) and torque loading (high load torque during start is worse). You can roughly roll up the effects of inertia, loading, and voltage into starting time. Motors with a longer starting time have a more severe start than motors with a shorter starting time, and deserve more frequent monitoring, all other things being equal.
These comments apply only to direct on-line start motors. VFD have a much easier start.
A final point on rotor bars, I believe it will typically give a long warning before it completely fails. The most likely scenario for rotor bar degradation will be increasing vibration, possibly increased starting time (which are not desirable but generally do not make the motor unavailable). Unless a rotor bar comes completely out of it's slot or starting time degrades to the point that the relays take the motor out during start, you will likely still have use of the motor for a long period after detection. In contrast, a winding failure removes the motor from service immediately.
These are factors specifically related to rotor bars. Of course in deciding any PM or PDM, you have standard considerations of motor criticality, availability of a spare etc etc.