First, the slot being covered is far from a certainty. The slot is currently set to be manned by either a rookie or the return of an injured player.
Second, of course the CB2 position is a glaring need, as of now. We know Rowe has been a dismal failure at it to this point in his career, and we know that McCourty hasn't shown much of anything on the field since he's been a member of the Patriots. Now, we can play the game of HOPEANDPRAY! like we did with the DL and LB groups last year, and the Patriots will almost certainly still make the playoffs because Brady/BB, but we should at least acknowledge that we're playing that game, and that having to play that game will probably bite the team in the ass at some point.