The point is, it sounds good to say "we need to get an immediate starter at CB"...but in reality regardless of who we pick, they will likely be the dime back or inactive their rookie year.
Realistically, rookie CB's start about as often as rookie QB's. While I would rather take a DT at 10, your reasoning is why I would rather pick DRC than McKelvin. He has more upside with the rare combination of speed, height, long arms, and leaping ability