The only sort of negotiation worth "winning" is one in which there is no ongoing relationship at the conclusion of the negotiation, i.e. certain types of purchases/sales such as for example a house or a car.
If there is an ongoing relationship and one party "won" the negotiation then the price is resentment and likely "evening up" by the "loser" at some point in the future.
In many cases it is worth conceding some value to the other party even if the other party has no leverage just because you (as the "winner") want the other party to feel that they did not "lose" the negotiation. For example say a potential employee asks for an extra $10K. The employer might know that the new employee will take the job even if the $10K request is refused. But just to ensure the new employee saves face and feels like they were valued and listened to and did not "lose", then a wise employer might give them $1K or $2K or some other concession instead such as a guaranteed review in 3 months.
I totally agree with you, under a lot of circumstances. Many job-hunters/HR reps treat the interview process as a one-time deal. Therefore, it is my responsibility to prove my worth. When I negotiate a higher salary, it's not because I want the extra money (though it is a factor), it's because I want to prove to them I am worth at least that much to them (hopefully more).
At the end of the day, unless I'm a terrible employee, the employer still profits off of me and still "wins." There might some some opportunity cost here and there (they could have hired person B for $X less!), so from there on out it is my job to prove that I am worth the $Y salary that I worked so hard to negotiate. I don't want them to think about the "what ifs" of not hiring me, I want them to see the value I put on the table.
For me, personally, I always try to do my best work, no matter how much I am being paid (my latest contract job was $15/hour and was definitely way below my pay grade, but I put my best foot forward regardless).
Since I know that many other people will only perform to the rate at which they think they should perform at that pay level, that puts me at an advantage. I can negotiate a higher salary and prove that I will always over-deliver, thereby proving my worth.
I get a nicer salary, they get an exceptional employee, win-win.
Of course, if I end up not performing up to the price I put on myself, then the company is not getting what they are paying for, and I'll probably find my name on the list of lay-offs when it comes to that. Rightfully so, too.
If there is an ongoing relationship and one party "won" the negotiation then the price is resentment and likely "evening up" by the "loser" at some point in the future.
In many cases it is worth conceding some value to the other party even if the other party has no leverage just because you (as the "winner") want the other party to feel that they did not "lose" the negotiation. For example say a potential employee asks for an extra $10K. The employer might know that the new employee will take the job even if the $10K request is refused. But just to ensure the new employee saves face and feels like they were valued and listened to and did not "lose", then a wise employer might give them $1K or $2K or some other concession instead such as a guaranteed review in 3 months.