Change my example from barter to kittens. Proceed. :)
The point is still the same. The game can be broken with a gotta catch 'em all mentality. As the book says, I don't recommend it.
"Hey little girl, if you do what I want, you can have this kitten."
"Fuck your kitten, there ain't enough meat on it!"
Is a kitten leverage? Well, the MC looks at his NPC and follows his principles, specifically the one where he names people, and makes them human. Page 112 details all this.
So wandering down the street offering the same thing to each person isn't going to work. Manipulate works when you offer something that someone actually wants, or have some sort of information on someone that you can hold over them. People are different, they want different things.
Now, if this player is going to go up to a person, finding out about them so that they can get just the right thing to manipulate (reading a person is good for this), then manipulating them, what's wrong with turning them into an ally? I mean, they've done the work on this person, and clearly they care about getting this person on their side, so then... awesome!
But here's the funny bit. All NPCs in the game have cross hairs on them. So, killing them is par for the course. Any one of your NPCs can be slaughtered, at any time, by crosshairs, or by the player characters. Hell, some of the players can do it even with a weak hit from the hard moves (or act under fire if it calls for it).
So, Killing NPCs is really easy. So when a player decides not to just slaughter someone, and interacts with them instead, and finds just the right buttons to push, why not have them, on a 12, make that human connection?
I mean, everyone in the game is fundamentally human, even the grotesques in the game (it's in the rules!). So in this horrible world where life is cheap, choosing not to just point a gun in someone's face is a bold statement, and if a 12 is rolled, I think it makes for an interesting world when the spark of actual human decency occurs.
The die roll creates the uncertainty, and it prevents any status quo from happening. You don't know when you'll touch the heart of even the most stone cold killer, but when you do, it's something special.