I'm generally against new specs and classes in general. I'd rather have Blizzard using their resources towards more unique gameplay and mechanics for the different classes and specs we already have than to make more of the same. Sure, it will be tanking with earth magic but when you implement something like that into the game it doesn't really change anything. Unless there is a good way of making Shaman tanking a unique experience, I'd rather have Blizzard putting their resources elsewhere.
Besides that, I think the way the roles are devided at the moment is fine. Not all classes need a different type of spec next to dps. Let alone two!