This might explain a little more about the Devin McCourty pick:
"Chiefs GM Scott Pioli and head coach Todd Haley were talking about the selection of Javier Arenas, who will essentially be the third corner. He was the 50th overall selection in the draft which, on the surface, seems a little high for a part-time starter.
"The reality is, we studied this and most third corners on a team are now playing an upward 55 percent of the snaps a year," the Chiefs GM said."
So yes; it's all about the snap count.