Hypersmurf said:
That's wrong.
"You cannot spend so much XP that you lose a level, so you cannot cast the spell unless you have enough XP to spare." PHB p151.
It's not wrong.
It's a house rule

. Like I said, the level system shouldn't be so obvious in the game. Some characters should be identified as more powerful, but there should not be be any idea of the game characters about the specific time a level is gained.
Hypersmurf said:
That's right. But it doesn't always work.
Sometimes the character really wants something to happen, and he rolls well first time - no need to expend the Luck power.
Sometimes the first roll fails, and the second roll succeeds.
Sometimes the character really wants something to happen, and both rolls fail. Luck power didn't work.
Sometimes the first roll fails, but the player has already expended the power for the day.
So all the character knows is that over a lifetime, he has slightly better luck than other people. "Once per day" is a metagame concept, just like "one AoO per round".
On average, over the course of a year, a spiked chain hits people more often per combat than a longsword does. But the characters won't describe it as "he already made an AoO this round". They just know that it's dangerous to charge in incautiously against a glaive or spiked chain.
-Hyp.
You aren't always lucky, that is the nature of luck. The character does understand that when he really wants something to succeed, he has a much better chance (about double) than he normally does and that he can't rely on this more than about once per day. The character understands that this only kicks in when he "really wants" something, not on a purely random basis. So, the luck domain exsists as an in game concept - the cleric can "press his luck" as it were.
AoOs exsist as an in-game concept as well.
Example: A very strong fighter is guarding a door that the PCs need to get through and fast. The fighter is ready for combat. In a metagame concept, all the PCs delay until the highest AC/most HP character goes up, past the NPC and opens the door. He expects to be hit, but he can take it. The rest of the PCs then move through their friend, through the open door, and on to their objective.
How could the characters understand this? Under the "wild swing" theory, I don't know. We agree on the rules, but what about the explanation of the in-game action? If the PCs do not understand these tatics in-game, they would be down a very useful tatical option, which isn't right. The rules allow for it, so the characters in the game should be aware of it.
I can think of many examples where one side or another will attempt to cause some character to waste their AoO so they won't use it when it is much more useful. The provoked side can choose not to use the AoO to prevent this. But how do you choose or not choose to use a wild swing? In character it makes no sense, so in character tatics would suffer. Any characters who tried what happened in that example then aren't being clever or inventive, the are metagaming and being bad roleplayers.