I'm saying that the real mechanical benefit of making an alliance (or engaging in subterfuge, whatever) should be equivalent to the real mechanical benefit of fighting. If fighting gives you 500xp, then the benefit of not-fighting should also be worth ~500xp. But in a system where levels measure how good you are at fighting, and XP is awarded for fighting, the mechanical benefit of not-fighting would not be XP. Instead, the benefit would be that you don't waste resources (such as HP and spell slots).
Imagine two possible paths for Jim the Ranger:
Going down Path 1, Jim kills everything in his path. Two hours later, he has earned 500xp, found 400sp, and has a +1 longsword. But, he's also down to 12hp, and only has one spell slot left.
Going down Path A, Jim avoids confrontation, through a mixture of stealth and diplomacy. Two hours later, he has earned zero xp, found 100sp, and doesn't have a +1 longsword. But he still has 100hp and 7 spell slots left.
At this point, Jim comes across the Big Bad who had orchestrated this whole scheme, and they aren't willing to negotiate. They are going to battle with Jim, to the death.
If Jim 1 wins, then he goes home with the grand prize: everything he looted earlier, plus XP and loot from the boss, and he's saved the day.
If Jim A wins, then he goes home with a lesser prize: some XP and loot from the boss, plus 100sp, and he's saved the day.
If either Jim loses, then he gets the consolation prize: death, and none of the loot or XP from before.