1. Metagaming in a RP context does refer to "player thinking about what character woudn't know", I meant it in a game design context where it is a seperate game within another game, e.g. any resource management system. Should've used a different word, but the point still stands that when you add something like this it should increase the decisions you make, not decrease them.
2. Whether you need them to win: I agree that this is a relevant choice that adds to your decision making, however as you've said, you very rarely "need" them to win. Instead a lot of the fun comes from using the spells in ways that synergize with your teamates or as part of a battle strategy, which you can't do because you might need them later. There's plenty of game decisions or strategies I don't need to use in order to win, but part of the fun is in trying to be efficient and explore the intricately designed battle system.
3. Judging the situation: I think I've argued that the binary "do I need this to win" is not a sufficient addition to decision making to offset the limitations it creates in strategy. Furthermore, based on what information are you "judging the situation"? You don't know what's waiting for you over the bridge or how far away the next rest area is. It would be a relevant choice if you had anything to base it on, but you don't so it just becomes easier to not use spells. After a whiule you could begin to guess how far each checkpoint is, or during a second playthough you would know in advance, but other than that you're judgement isn't based on anything because you don't know whats up next, you're exploring.
4. Spells are too good to be per encounter: I totally agree with you on this and I'm not suggesting they patch this change, but the spells are that good because they were designed for casts/day, not the other way around. My comments are about the issues in decades-old game design that we don't really look at because...nostalgia? I have suggested ways to improve the resource managment metagame and I think any of them would be better than the simplistic limitation used here. This system reduces your incentives to make meaningful decisions in battle and that's bad game design.
Again, to be clear...really enjoying the game.