I am involved in discussions about balance of the Force for years and finally i have some explanation which i think is the real answer.
The Force.. We know that it divides into two; the living and unifying force "that penetrates us and binds us together", respectively.
The path called "Dark Side" contains some abilities which have the effect of creating unbalance on the unifying Force, "some consider to be unnatural". That's why it is "Dark", that's why the Jedi don't follow that path. And that's why the Dark Side is more powerful, as George Lucas stated in Trilogy DVD Documentaries. Let's look at some quotes from the last movie:
obi-wan: you were the chosen one! it was said that you would destroy the sith, not join them. it was you who would bring balance to the force, not leave it in darkness.
obi-wan: with all due respect, master, is he not the chosen one? is he not to destroy the sith and bring balance to the force?
The existence of the Sith and their usage of the Dark Side, creates an unbalance on the Force, because of the nature of the Dark Side. If the Sith is destroyed, this unbalance ends and the Force gets back into balance.
If 2 sith - 2 jedi setting is the balance, why would then jedi have ever wanted it?
Anakin Skywalker, the chosen one, kills the Emperor in the 6th movie and brings balance to the Force.