The Crimson Binome
Hero
I'm fully on-board with motivating the PCs rather than the players. Meta-gaming is bad. Maybe I'm just not getting my point across.There were times in college when I pulled all-nighters. But that was because I procrastinated or was taking too many credits. That doesn't mean cramming is the best way to learn, many studies have shown that it's one of the worst.
I think changing a basic assumption of how we learn and advance skill is incredibly ham-fisted and transparent way of trying to justify motivating the players, not the PCs. If you want to take the route of motivating the players to get a desired in-game result, just say so. I'm not going to judge you. I'll just disagree.
Trivial encounters are trivial. A level 1 character learns nothing from stepping on an ant, and a level 17 warlock learns (effectively) nothing by blasting four goblins before any of them get a chance to move. You learn more by challenging someone who has skill equal-to or greater-than your own, than you could learn by challenging someone whose skill you already exceed. It's true of fencing, tennis, and chess. That is an aspect of real-world learning which the game happens to model sufficiently well, and we don't want to sacrifice that.
If you're a level 9 warlock, and you can cast two fireball V spells (per short rest), then your first two encounters against six orcs (per short rest) are trivial. You learn nothing by killing them all before they have a chance to move, and that's what my hypothetical learning curve is supposed to reflect. In the same way that you learn more from fighting a beholder than you learn from fighting an ogre, you learn more from fighting without access to your best moves than you learn from fighting with them.
Granted, putting the cut-off at exactly four encounters might seem pretty artificial, but it's just to demonstrate the general point. A better-balanced version of the rule would give the DM discretion over whether or not any given encounter was trivial. You could also say that all encounters are worth less XP until such point as the listed XP would push you over the threshold of half-expected value for the day, at which point they start awarding more; but that sounds like a lot of work for very little payoff.