Buffing is not Teamwork
Buffing is not teamwork. Buffing is delegation. The spellcaster doing the buffing is transfering their own powers to the buffed character (who, let's face it, it's a fighter or rogue or someone else holding a sharp object) instead of taking actions themselves. The party becomes a micro republic: the spellcaster has elected a representative, and that representative is going to go forward and get things done while the spellcaster sits back and waits.
To be fair, it is not the transfer of power that makes it a problem. It's the ratio of actions. The buffer casts spells before the fight or early on, usually dropping a few buffs and then waiting while the buffed character takes actions. Every combat round that the spellcaster has nothing to do while the buffed character is fighting with his newfound power is a round that was delegated.
The problem is the spellcaster doing the buffing is actually playing less. If the spellcaster had to cast buffs every round just to keep up (such as if the buff only lasted one round), then the spellcaster could buff but still be a participant in the combat.
Sure it's rarely that clear cut. Even if the wizard has used all his powerful spells to buff the fighter, there are always magic missiles to be cast, right? Sure! But the power transfer means that the spellcaster's direct contribution is going to be weaker than if the spellcaster didn't buff and had spells of his own to use.
Some players are perfectly happy delegating. That might be their whole strategy (“Save me Wyck!”). There's not a thing wrong with that. Some players don't like tactical combat either, so this could just be their way of helping out but then not having to deal with the details. That's fine.
Most players do not have this attitude and they get pressured into buffing. First it's only on special occasions, but after you've done it once the sharp object crowd certainly wouldn't mind having it all the time. As they sit quietly waiting for the spellcasters to prepare spells, they can't resist the urge to throw in a little suggestion that a barkskin or two gets on the list. Yes, bull's strength is a gateway drug. The spellcaster is soon put in the position of defending their own character choices, arguing why they aren't showering the whole party with greater invisibility (“But it would be so cool!”).
So the spellcaster buffs, and during combat the spellcaster doesn't want to just sit there so the spellcaster casts direct effect spells too. Because of the buffs the fighters have an easy time, the fiery breathing flagon goes down quickly, and everyone cheers and is ready to move on. Except the spellcaster is now drained of spells having cast buffs before the fight and levin bolts during. Weak in mana. Lacking eldritch energies. Low on the witch-o-meter.
The fighters are fine (having taken less damage because of the mighty buffs they received) so they want to press on. The spellcaster can just accept her fate and lurk in the middle of the marching order, basically doing nothing for the rest of the day, or press for a halt and camp. Nothing more comforting than a camp fire at ten o'clock in the morning.
Likely other players will grouse and blame the spellcasters for the inexplicable short day, but they are forgetting it was the buff that got them here in the first place. They are healthy because they borrowed the buffer's power.
And if the spellcaster does just hide in the marching order and endure being powerless the rest of the day they get to play even less.
So isn't healing a lot like buffing after the fact? The healer uses up spells to bring the fighters up to full strength instead of preventing them from losing that strength in the first place, losing his own spellcasting power in return.
Giving clerics the ability to drop prepared spells for healing (Third Edition D&D) means the healer doesn't have to waste slots “waiting” to heal, but it also means spells used in combat cannot be used for healing later, so the cleric is pressured to conserve spells to benefit the fighters later on.
A cleric is by definition something of a buffing class, including spells that benefit the whole party. It is often more effective for the cleric to cast spells to prevent damage than to heal it afterwards. But the cleric also has reasonable fighting abilities, so a cleric with no spells can just switch roles and be a fighter, making the cleric a little more resistant to the buff trap.