Manipulation is done all the time. The media, your boss, school system and the government all have their hand in pointing you in the direction they want you to go rather than where you want to go.
I think manipulation is the common thread of every abuse and every crime. It is hard to always be 100% honest about your motives 100% of the time, as well as 100% respectful of the decision-making rights of others 100% of the time. But I think the more we do that in our personal lives, and the more we encourage it in our leaders, the more we'll be the society we wish we were.
If we are manipulating people to get what we want then it means we have no reserves about what we will do for ourselves. Ethically, people should make a conscious choice in their interactions with you. They should not be tricked.
Depends on the degree of manipulation. If you manipulate someone into being your friend so you won't be lonely is not the same as manipulating someone into putting your name in their will. I think people should be flexible and should have the skill to manipulate in order to achieve ones goals.