Personally i think that the Force is naturally balanced, but each time someone uses it (for good or evil) it unbalances it - which is why we end up with the 'dark' and 'light' sides of the Force. Without the Sith to counter the Jedi, the Force would become massively unbalanced (and vice versa). Sith usage of the Force is stronger than than that of the Jedi, which is why there are more Jedi than Sith. The destruction of the old Jedi Order by Anakin was essentially to wipe the slate clean, returning the Force to natural balance.
I know it sounds weird, and it might not fully make sense (i only fully organised it while posting it now), but...