You are missing the point of Dr. Manhattan he is a GOD, by all intents and purposes especially considering the fact no one else on Earth possesses any powers. He usage in the characters is to show that a human with no particular differences or anything really standing out other than a scientific educational background receives powers of extraordinary amounts. Once receiving these powers he not only aligns with a particular country which is completely reverse of the typical Superman being a world savior but on top of it has the ability to continuously disarm other countries, get involved in wars and decide the victors yet does not get involved until asked. Not only that but he continues to become more and more disassociated with humanity. Manhattan could essentially make sure war is an impossibility but he doesn't. He is involved with Veidt more for the work aspect then the outcome. He shows the essential question what happens when God turns his back on you? What happens when a God decides not to intervene in human affairs except on a superficial level. Superman gets involved in every aspect of the world's affairs, he is dying to be apart of humanity. Manhattan who is already a part of it simply chooses to ignore it.
The most powerful person on Earth has no desire to intervene, even in the end game decides that a charade is more important than taking the reins and becoming even an Emperor? His non-involvment speaks more volumes than anything he actually does. Its an excellent and thought provoking thought device.