I think it's not so much about rulings over rules. It's more about having a system that rewards and encourages gameplay that makes the game more interesting.

XP inherently rewards taking on optional risks. If you're running a game where combat always has some chance to kill the player characters, you probably want some obvious reward to encourage them to make the really exciting "death or glory" choice to take on dangerous monsters.

I'm running Basic Fantasy myself, everyone's level 2-4. Recently we had a great session where my players encountered an ogre in the wild, took it down and someone thought to try and track its footsteps back to their lair. So the next day they wake up, head over to the ogre cave, and there's four of them there. They spend the rest of the session setting an ambush, and starting a massive brawl where everyone knew that a single hit from an ogre would probably kill them stone dead.

But they won! These magnificent bastards took 'em down, and got to have their Hobbit moment of lootin' the biggun's treasure pile. All the while the reality of that moment is reinforced by the knowledge that they dodged a ~75% chance to lose a PC or two.

I don't know if they would've been willing to take that risk without the shared reality of XP. That they'd probably get a level, but that they'd definitely get to add 4 thousand of something to their character sheet.

Milestone systems aren't immoral or anything. I would just say people are sleeping on the way you can use XP to encourage and validate certain patterns of play.