Err what? You mean the idea that DO can only control a limited amount of things at once? That would not prevent the DO from influencing the WT, then the BT then Caemlyn and so on, depending on where his influence would do the most harm. Having SG-stone in all major cities would allow the DO to choose which place to influence, and change the location of that influence at will. Right now for example, the White Tower would be a far better place for the DO to focus his attention on than the Black Tower.

You are probably right about it being just symbolic though. It seems unlikely that Sanderson would have told us if its going to play a role in the coming book.