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Because the curve is the hardest to locate in baseball, if you don't have it on a given day, you're usually dead meat. It's why Boomer Wells had a top notch change-up, Roy Halladay had the sinker/change attack he could use, K-Pax has his cutter, Mussina had the change/slider, and Aaron Sele had a 4.00 ERA most years (because he didn't have an alternative and therefore blew up about one time out of five).
Karns should work on a change-up or something because otherwise he is going to be maddeningly inconsistent.