Why does a character get Fate points from invocation only at the end of a scene in Fate Core?

Question

This answer to "What happens to the fate point after a character invokes an aspect?" shows that in DFRPG (per Your Story, p. 106):

if you're invoking an aspect on another PC or on a NPC to gain an advantage over them, that character will receive the fate point you spent, either at the end of the exchange (in conflict, see page 197) or at the end of the scene (outside of conflict).

But in Fate Core (p. 81):

if someone pays a fate point to invoke an aspect attached to your character, you gain their fate point at the end of the scene.

Why did Evil Hat change invocation so that Fate points are given out only at the end of a scene?

Answer 1

Because it discourages zero-sum exchanges.

Someone just asked this of Fred Hicks in the Fate Core Kickstarter comments. He replied that if Fate points were exchanged immediately after the action in which they were used, the flow of Fate points would become zero-sum:

Let's imagine we're doing it your way. I have an aspect you're invoking, to my detriment, so I'm gonna be getting that fate point.

You spend it and I get it right away.

I take what I got from you right away and spend it to invoke one of your aspects.

This cheapens the use of Fate points because you can get them back so quickly and easily, and compromises the flow of the narrative crisis/victory cycle that is the hallmark of the Fate point economy. A failure due to being invoked against shouldn't set you up for success in the next exchange, but in the next scene.