Ugh, it's as if nobody ever reads anything that is posted on here. Let's get some things out of the way.
1. "The BG series did pre-buffing and resource attrition perfectly so there was no need to change anything!"
No, it didn't. You always, ALWAYS had the option to rest and recharge whatever you wanted, effectively removing any sort of resource attrition that might've been intended (which we know it wasn't, BG was a dumbed down game for casuals). There wasn't a single fight in the whole game which required you to empty your entire spellbook to force you to think whether to use the spell slots for buffs or damaging spells. Both of these things combined means the Vancian system was mangled and wasn't used at all for the purpose it was created.
2. "Nobody complained about pre-buffing in ye olden days!"
Even if this is absolutely true (which it isn't) and nobody ever complained, that doesn't mean it's not a degenerate mechanic that only serves to waste time and only ever truly rewards metagaming even if done perfectly. If done perfectly, you wouldn't be able to scout out a fight, change the spells, rest and then do the fight, you'd be locked in your choice for a duration of time/the whole dungeon, so only metagaming can ever be rewarded. When there is no pre-buffing, at least you get the knowledge to not use that spell you've memorized in that fight when you scout and save it for later. Sawyer might be the first person to ever notice it and that wouldn't make it less valid.
3. "It's action economy!" (in the sense that it frees you to do other stuff during combat)
There are other ways to eliminate buffing to let you do other things in combat, whether that is permanent auras or feats, lack of buffing in general, permanent buffs you get from quests, etc. There's no need for it to be a time-waster the way it is now. It also removes a defensive layer in combat, you can only ever use your spells offensively if this is taken to the logical extreme. The exceptions being healing spells and if a buff runs out in combat, which people praise, but that can be applied to all buffs to always add that bit of choice. Which leads to -
4. "It adds a tactical choice!" (and it's conspicuously only when a buff runs out in combat)
I don't know why people do this, purposefully ignoring their own arguments. You praise the addition of tactical choice, but condemn it when it's actually present. It's only ever a choice when there is another, equally valid, choice to make in its place. That means that you EITHER buff OR do something else, not have the option to do both at the same time, which pre-buffing allows. The counter argument here is that it's resource conservation to choose to not use the slots for buffs, but that is not an inherent pro in pre-buffing. You can still have spell slots AND no pre-buffing, like PoE showed. Let's transpose this to a D&D game with no prebuffing, you choose Armor, Burning Hands and Blindness, so you are now free to cast those spells in combat. You cast Armor in the first fight (instead of before the first fight) and bam, that spell slot is already used. You could've chosen 2 times Burning Hands instead of Armor. There is no argument here because this can be present when there is pre-buffing and when there isn't. I.e. this is irrelevant.
5. "You can choose not to pre-buff if it's that tedious; or there is no need to pre-buff for every fight, so there is no tediousness involved!"
I didn't know not using the system to your fullest advantage is a virtue. I really don't know what else to say here, the argument is basically that it is tedious but you can make it not tedious by not using it. How is this a defense and not an indictment?
People only like pre-buffing because it makes them feel smart compared to people who don't pre-buff. Like they've found the Holy Grail. It's a shiny veneer that seems intelligent because it makes you win fights, but in actuality it enabled everyone to win by increasing the stats of your characters to such high levels before combat that it's almost impossible to lose without actually having to do any thinking or make any erudite choices.
If you have any other arguments, come at me.