Sigh! There is a huge difference between manipulating someone and negotiating to get what you want. The first implies control, generally without their consent. The latter is all about understanding their needs (which are often emotional and may require asking probing questions, reading body language, making that person comfortable etc) and finding out how you can come to a consensus that satisfies both parties.

The problem I was alluding to is that technical people seem to think that using anything other than cold, hard facts to come to terms is "manipulative." And quite simply, the world doesn't work that way.

[edit] Now that I re-read your reply, I think I took it completely the wrong way :-) Sorry.