Dernbach obviously isn't good enough, and shouldn't be in the side, but I don't think he's that bad. He often bowls at the worst times for bowlers, at the start, in the batting powerplay and at the death. He just needs to stop bowling his slower ball so often, use it as a surprise delivery, not your secondary weapon. Yorker, length, and mix in the slower stuff. He's technically pretty good, the slower ball comes out beautifully, he just doesn't have the bowling intelligence to know when to bowl the right delivery. He's a much better limited overs bowler than Mahmood, Plunkett and Onions though, and certainly not the worst ODi bowler of all-time.