Yes and no. He needs the CC to land his damage. However, turning enemy ADC into a limp noodle meanwhile is simply not necessary. He can force the 1v1 (I guess), but at least let the guy fight back.
The defense against him is basically to wait out his overwhelming ultimate buffs or magically CC him down as a 5-man. His counterplay to that is to pick a unit and make them helpless to be picked off. However, to make that unit utterly useless meanwhile is not necessary, especially without possibility of dodging it.
Either the attack speed cripple needs to go, or that slow needs to lose a little of its bite. It does not need to be that strong, especially since CDR is core on him already. The downtime ends up being maybe 1.6 seconds, almost down to 1 second with Cosmic Insight.
He's supposed to be a snowball-reliant champ. With Wither's power, it requires a team effort to keep that from happening unless directly countered, and even then, he can farm under tower plenty safely with ease.