Most people who are passionate enough about something to receive these fellowships will take the opportunity to pursue their passion.
I had a Shuttleworth Fellowship for 2 years which is very similar to the MacArthur Genius grant. They are both often enough to not worry about having to work a job to pay your bills and feed your family. (The Shuttleworth fellowship includes enough to pay for legal, financial, travel, contractors, etc)
I worked harder during my fellowship than I can recall otherwise. I knew it was a once in a lifetime opportunity and that I only had a finite amount of time to do something great.
I wrote about my experience as a fellow here.