It can only be a good thing to have a tank. Just because you have it doesn't mean you have to always use it. Keep the 5 shaman for PVP if that works better for you, and swap out one shaman for a tank here and there when needed. Sure, you might spend a lot of time leveling when you have to now bring a 6th character up to 80, but if you do it I'm sure you'd find the tank useful to have in PVE content if you ever happen to want to run a 5 man for something.