When the lights pull 200mA each, and you have a garden where you're paralleling lights for a total distance longer than 10 ft, it very well could be a concern. (Granted, in this case I would advise using better than 24awg cable.)
but they do response bio-chemicaly to energy levels, which are effected if you use PWM. using PWM LED output may still offer some charming results to efficiency in your application.
I don't think it's for the plants. Why would grow lights need motion detectors?