Originally posted by Moses
What teams do is they play their best man-to-man corner at the LCB spot. Almost every NFL team's best corner is the LCB, with a few notable exceptions like Detroit [Dre Bly], Buffalo [Nate Clements], Green Bay [Al Harris], and [I think, but I'm not 100% sure] Seattle [Marcus Trufant]. The LCB is lined up over the opposing team's Z receiver [most teams' #2 receiver] and is given the task of shutting him down in single coverage.
With the LCB shutting the Z down by himself, it allows the strong safety to play the run more agressively. In turn, the RCB and free safety double cover the X receiver [usually the number 1 receiver].
Quite frankly, the demands of playing LCB are far, far more demanding. Al Harris came to the Packers as a nickleback in Philadelphia and took over the RCB spot because Mike McKenzie was firmly entrenched at LCB. Common sense dictates that if Harris was able to play LCB, they wouldn't have spent a bunch of draft picks on highly athletic corners [LCB prototypes]. RCBs are easy to replace. LCBs aren't.