Considerations For Motor Cables: what are the downsides to ribbon cable?
array of motors ... under almost no load


Depending on the motor, even under low load current draw can be substantial. The current draw will of course increase under load, more so in stall conditions.


  using flimsy old ribbon cable to supply power to a motor is a bad choice


Not necessarily true: Inside your CD ROM drive you may well find motors with a full load current flowing through FPC / FFC connections along the way, even flimsier than a ribbon cable. It depends on the current drawn and the rotation and PWM frequencies (where applicable). 

Also, hard disc drives (not SSDs) have motors with substantial spin-up current requirements, and their supply current comes in via the same ribbon cables (IDE cables, for instance).


  is there a compelling reason beyond mechanical concerns 


There is, if the motor is being controlled via a very high frequency PWM signal, or is itself a high speed motor: The signals would radiate EMI, if not shielded - The recommendation in such cases would be shielded twisted pairs, at the extreme. However, for a hobby project such a consideration is most likely maximum overkill. 

Conclusion: If the ribbon can handle the current, nothing wrong with using it.