For high power, that seems a very expensive way to do the job! Converting to DC and back to AC ain't cheap for high power!
Still way more reliable than some type of variable speed drive gearbox. Generally a VFD control box on on one of our pivots will run about $350. I've only had to replace one in the last 10 years. Considering that I can loose up to $500 per day of down time on a single pivot due to yield loss, yeah they're cheap.