Speed is only necessary in the NFL if you want to be a playmaker. Good example of this is Phillip Buchannon on the Raiders. Doesn't make him a top or even above average NFL corner though, but he will probably score you a touchdown or two every season..
Teams draft for speed because they want to have a CHANCE at landing themselves that Deion Sanders/Ed Reed type player. Most of the time it's just another Phillip Buchannon.
Antrell Rolle will never be that type of guy, his 40-time is a reminder to all NFL GMs of that.