I too am quite surprised that Clarke wasn't picked but, honestly, until the Dutch Open, he really hadn't been playing that well. He showed well at Firestone, but Poulter and Casey have performed better over the last year.
Personally, I was hoping for Clarke and Poulter. Between Clarke, Poulter and Casey, if I had to bet on a player to hole an important putt in the Ryder Cup, Casey would be the last guy I would choose.