There. I said it. I'm an edition warrior. Getting experience for killing monsters is intrinsically bad.
Hyperbole? Sure. Right? Absolutely.
Here is what I have been noticing: as player's increase in skill (which in 3+ D&D largely means character optimization and ability on the battle mat, although my players are starting to pick up on my traditional D&D tricks a bit too) it becomes increasingly difficult to pose appropriate challenges to them. It seems that most fights wind up being one said pummeling the other.
Okay, you say, but why does that matter? I thought we OSR folks hate the idea of the "appropriate challenge?"
True! But therein lies the problem. In 3+ killing monsters is the only way to gain experience. That means that to progress players must figure out ways to kill the baddies. If I am doing my job as a DM they have to use their brains to kill them, but kill them they must. This means that player's have a big incentive to try to make their characters as combat worthy as possible. So if I place a monster that is truly too difficult for them I am basically cheating them. It would be like placing a dungeon with zero treasure inside in traditional D&D. In fact it is slightly worse, because I am not only wasting their time but I am going to retard their progress. An unkillable monster is much worse than a normal red herring because it slaughters the PCs as well.
Actually after writing that I a have come up with a better analogy for traditional D&D: imagine a huge chest brimming with gold and jewels. The chest is surrounded by horrific traps that you, the DM, know the PCs have basically zero chance to pass through alive, no matter what they do. We also know that the players will spend enormous amounts of time trying to get that gold. They will spend hours of real time and more than a few characters trying to get it. This is an unfair situation. Figuring out how to get that gold is exactly what we have told them they should be doing, and in fact it is heavily incentivized. They will be frustrated and confused if there is truly no way for them to get it.
Likewise, in 3+ they see a big fat monster, dripping with CR. We have told them that the purpose of this game is to kill as many monsters as they can, so of course they try to kill it. They can try to kill in a smart way or a dumb way. It might be a tough fight, it might consume many of their irreplaceable resources. It might even kill a few characters. However, if the fight is fundamentally unwinnable for some reason (monster is simply too tough/has too big damage output, damage resistance, nasty magical abilities, etc.) then we have cheated the players.
When x is the objective of the game, we should not create situations where going for x makes you lose. Sure it can be hard, there can be tricks, traps, and pitfalls. But the goal of the DM should be to create fun for everyone. Frustrating the very objective of the game for players is not fun.
That being said CR sucks. I am not going to spend a lot of time beating that dead horse, but suffice it to say that the idea of adventurers going out and encountering a series of fights carefully tailored to their ability levels is just dumb. Also Conan runs away a lot.
So awarding experience for monsters slain makes CR matter.
CR is badwrongfun because it is stupid.
Therefore, experience for killing monsters is badwrongfun.