What will motivate me to provide an isolation boundary in a motor controller?
In a modern AC motor drive such as depicted in that document, the input power supply passes through a full-wave bridge rectifier and is used to charge a high voltage DC capacitor bank from which the drive electronics then synthesize variable frequency and voltage 3-phase waveforms.  Higher power industrial versions may rectify a 3-phase input, but the idea is the same.

Without an isolation transformer, the lower side of that capacitor bank will not be at ground potential - rather, the capacitor voltage will normally be approximately symmetric with respect to ground.  Needless to say this requires care to work on!

The size, weight, loss, and cost of an isolation transformer increases with power requirement - but the cost of a mechanism for isolating the control circuitry is basically fixed.