IMO, the rationale from the PHB is rather flimsy. Daily powers are the intrusion of metagame conerns into the story. Yes, yes, it does create a sort of illusionary balance that seems to pacify players that want win against the "monsters".
Some in-chracter exchanges about daily powers:
A:"Hey, use that trick that, you used to whack the orcs."
B:"I can't. I tapped my daily reserves for that, now i only can tap my reserves to run faster."
A:"??? ... Well, then use that trick you used to blind the gobin."
B:"I cant. I tapped my reserves for that, too. I can only use it again when we meet the next batch of monsters."
A:"???? ... Well tap your reserves for running faster, to what that goblin."
B:"But understand. I can only tap my resources only once in a specific way. It's a rule...." (BZZZT. Metagaming so we correct to) "its a universal law."
A: <sigh> "But I can tap my resources indefinitly to throw someone sand in the eyes."
B:"Dont bring up things that aren't covered in unversal law. You know full well that THROW SAND would be a power that the gods did not make available for us. And for it to be a fair power, it will tap your resources in way so that you can use it only once for reach batch of monsters."
A: <stunned silence>