The master must be a hobbyist.

Practice is the key to mastery (read the book!). Part of practice is accepting long plateaus. Progress is bursty and fueled by time spent plateaued.

When working towards mastery in a craft that is part of your profession, customer needs will dictate the lion's share of your craft's development. 

To subordinate customer needs to your craft is to abandon the path of mastery in your profession. 

To surrender all practice outside customer needs is to abandon the practice of your craft.

This is why the professional master must also be a hobbyist. Only as a hobbyist can you practice your craft for its own sake. Only as a hobbyist can you playfully experiment within your practice. Putting aside your craft as a hobby means your are mastering your profession but not your craft.

And that's why I need to go code some tank AIs now!