its not reliable enough. he can do what amumu does, but it requires some pretty slick moves before he can do it. if you've never played kennan before, its actually fairly hard to use him in the team fight phase. in order to get your stun go gotta get some good work in with your electric ball and that means you gotta be pretty close. your liable to get stunned or rooted if your trying to intitiate against a whole team.
malphite and amumu are the big aoe disrupter tanks and they both have moves to get them in close really quick without too much risk of getting shut down. The other tanks are more taunt or shield oriented so they are kinda a different sort of tank. however, off tanking with him is great cause if your enemy is focusing on your tank, or even your carries, its unlikely they are gonna be fast enough to take the time to shut down a blur of electricity.