partly because he decided that anything less would have been a waste of potential.

The Master says thanks ever so much, he's glad you see his 'potential.'

. . . And I have this mental image of the Master in bed for, like, a week, with this Gigantic Stack of the Doctor's letters, catching up on what the loon's been up to the past year. Probably balling each letter up and tossing it into a corner as he finishes. And completely ignoring Torchwood. They want him that badly, they can come and get him.