Some free form thoughts - might not all make coherent sense...
Does neutrality exist as a power in the game world? I would have to say yes. Others here have stated things, probably better than I can, about balance. There are forces trying to maintain a balance, but they are not noticed as much. Forces of Good/Evil/Law/Chaos are powerful forces actively and openly working to achieve their ends and they let everyone know it. The forces of neutrality work without calling attention to itself in a direct way.
As a comparison, look at political ideologies. Who do you hear about the most? People who are strongly left-wing or right-wing tend to be more outspoken. They see things as black and white, as us and them. They heartily proclaim their superiority while stomping upon the other side. They invoke passionate and emotional responses. People who sit in the middle on the other hand, have a tendency to want to quiet the emotions, the rhetoric, get past the provocation and just make things happen.
This is how I see the forces of neutrality. They want to pull together the good and evil, not to make everybody a happy family, but to make things work, to make things happen. They do not need things to be in "perfect balance," there is a lot of wiggle room, as long as things continue to happen they are satisfied. They do not call attention to themselves and their work for two reasons:
1) The majority of people sit somewhere in the vast grey area, the Forces do not need dedicated followers in droves to push their philosophy upon the world, it just is the natural state of the world. It has to be forced to the extremes, but will natually gravitate to the middle, or somewhere near it.
2) It is boring. How many people scream and holler that we need more moderation? How exciting is that?
Perfect order = no creativity, no innovation.
Absolute chaos = no systems, no civilization
Pure good = those darn chipmunks from Bugs Bunny cartoons, "After you." "No, no after you, I insist." Nothing gets done for fear of hurting someone or taking something away from someone else.
Pure evil = complete selfishness