I think something people miss about the maximum encounter day: if you have 8 Encounters of appropriate difficulty, and they last on the linger side (3 Rounds), that's a whole 24 rounds of violent action. Which is to say, about a minute and a half of narrative time. Spread over a whole day.
Even if you don't reduce it to minutes and just encounters, that's still 8 encounters spread out over 24 hours, or one encounter every three hours on average. With no published schedule for when those encounters will happen, the adds are in favor of your spell being wasted unless it's one of those relatively few instances where you are initiating the fight after having prep time. As far I'm concerned, if you scouted or what have you and created a situation in which you can prepare, you SHOULD do better.
They won't be spread out over 24. They'll be spread out over 16, since 8 of those hours every day are from a long rest. The chances of a "wasted slot" are thus much less than you are claiming. Plus, let's be real, it's usually very easy to anticipate when a combat encounter is going to happen.
Encounters happen at night as well. Not as often as during the day, but it can't be ruled out, so only foolish players will use up all resources before resting. Even spread out over 16 makes it 50/50 that you are just throwing a slot into the garbage by casting it in advance without knowing when the next encounter will be.
A Paladin has just as much of a chance of wasting a spell slot when it smites for overkill damage. Arguably far moreso.
A bit, yes. But it will be at least partially effective in that if will take out a threat that could be doing damage. Unlike Larry, Curly, Moe, Shemp, Abbot, Costello, Third Base and First, the elementals that took 1 minute to summon and lasted a useless hour doing nothing before vanishing.
Or those elementals can actually run into a battle and make it trivial.
Nah. What's good for the goose is good for the gander. Those elementals are fighting the elementals of the enemy casters who cast their spell more than 1 minute earlier in preparation for a PC battle.
OK, let's say I don't want to cast Conjure Minor Elementals on the off-chance they might be wasted. (Which rarely happens in my experience, usually I have a pretty good idea of when to cast it and except combat, but that's neither here nor there.) That's fine, I have the best spell list in the game. I can cast something else with the 4th-level slot like Polymorph, Greater Invisibility, Evard's Black Tentacles, Banishment, etc.
Just out of curiosity, why are you so focused on damage? Damage is only a portion of what classes contribute. Huge saving throw bonuses, healing and disease removal just to name a few.
