(This is a basically a copy-paste of this answer I already provided to the linked question, the question itself may not be an exact duplicate, but I feel this answer completely answer this question)
What the Jedi Order call "balance" is not the middle point between dark and light side, its the absence of Dark Side use :
Traditional Jedi were keen to keep the Force "in balance". They attempted to achieve this by destroying the Sith and denying the dark side—essentially "keeping balance" by restoring the Force to its natural state, as they viewed the dark side as "corruption".
- Wookipedia article about "The force" section "The Jedi Order" paragraph 2
Also, about the Chosen One prophecy :
The idea of balance of the Force, a
central tenet of the Jedi Order,
refers to the ideal state in which the
Force exists in nature, i.e. as the
light side. The presence of the dark
side corrupts and destroys this
natural balance, and the Jedi viewed
it as their duty to restore it.
- Wookipedia article about the "Chosen One"
Finally As from Georges Lucas's intention :
Many fans incorrectly assume that balance refers to an equal mix of both light and dark side users. However, as George Lucas explains in the introductory documentary for the VHS version A New Hope, Special Edition, this is not the case:
"[...] Which brings us up to the films 4, 5, and 6, in which Anakin's offspring redeem him and allow him to fulfill the prophecy where he brings balance to the Force by doing away with the Sith and getting rid of evil in the universe..."
In an interview, Lucas compared the difference between the light and dark sides as being like the difference between a symbiotic relationship and a cancer. A symbiotic relationship is one which benefits both parties and in which neither is harmed, whereas a cancer takes without giving back, eventually causing the death of both parties
- Wookipedia article about the "Chosen One", Section "Behind the scenes"
Update : On a side note, I just watch the French translation of "The Phantom Menace" with my son and the "bring balance to the Force" concept was translated to "restaurer l'harmonie dans la Force" (to restore the harmony to the force). This is one of the unusual case where the intended concept had a better translation than the original.