I have a programming job somewhat like that, except that I work full-time for (approximately) 9 months followed by 6 months unpaid leave during which I make comics. The people I tell about this arrangement usually say they would love to the free time but could not afford to earn less. Some would not mind reducing their expenses but are afraid to ask their boss because it could hurt their career prospects. In most jobs the idea of working part-time is unusual, does not bring direct benefits to the employer, and requires some coordination effort, so why would they do it? The one big upside for the company is the employee's higher motivation, but unfortunately this is not easily quantifiable. I was lucky to be hired by a director who was also an artist wishing he had more time to paint.

During the dotcom boom I had an interview with a software company founded by students where many programmers worked 60%. They had some coordination problems. The company went down in the dotcom bust. My previous job was with a company where everybody worked 4 days a week. They wouldn't let me work 3 days after they had tried it with one employee and were not satisfied with the results. Before I worked 3 days a week as an economist and it went pretty well.

There are other jobs where part-time is more usual or even the norm. Many artists become part-time teachers.