We need several good cb's.
Several? Diggs played pretty well for a rookie, the injury set him back some, with more games he'd probably have continued to improve. So there's one. Robinson was a rookie and played little last year, he was drafted, he needs a chance to show if he can make the grade. Awuzie, Lewis and Brown all need to be considered, ability aside, they all have NFL experience, that has value just by itself.
Cowboys' record of free agent picks is poor, I don't trust them to all of a sudden sign a great or very good cornerback, which you'd have to in order to get "several" besides the draft.
Still need a cornerback in the draft, but if you draft "several" you're greatly reducing the team's chances of getting a linebacker, safety, offensive tackle or defensive tackle, all of which are also a need. And you can't totally ignore need...