Synchronous vs asynchronous PWM modulation
Simplicity. 

The subharmonics being discussed in chapter8  of Mohan is due to sum-difference type effects between the control and the carrier IF they are not synchronous. 

In digital-based controls this is typically not a concern as these are internally managed. However in analogue controls or mix controllers, the possibility of such a beat exists. the condition is mf needs to be an integer


  Small mf (mf &lt;= 21) 
  
  eg... if the inverter output frequency an hence the freqnency of
  vcontrol is 65.42Hz and mf=15, the triangle wave frequency should be
  EXACTLY 15 x 65.42 = 981.3Hz