Keep your friends close and your enemies in the next office?
Priebus knows Wash D.C., and actually is a good pick to interact with the D.C. insiders. As you say, Trump can keep a close eye on him.
What I noticed, is that in Trump's acceptance speech immediately after the election results crowned him President-elect, is that Priebus looked very nervous as Trump invited Priebus on stage and Trump thanked him for his efforts. I think there was probably some kind of deal brokered between them for mutual support and Priebus was nervous about how Trump could be so gracious. Just goes to show us that Trump is an astute manager, tapping his enemies potential for getting things done.