Seems just as tedious to have to activate the same set of buffs at the beginning of every fight.
Wrong. If you can pre-buff, you'll give your characters every little help to steamroll (and trivialise) the upcoming combat. If you can only cast buffs in combat you're only going to choose to cast those that actually make an impact, and adds strategical value to every encounter. Every spell cast uses precious time that you're going to think very carefully how to spend.
Some fights and especially the hardest ones (hi Adra Dragon) would be trivial and utterly boring if you could pre-buff to be protected/immune against certain effects off the bat and could go straight into casting heals/nuking spells.
The game is balanced around the premise that characters don't start combat buffed, and I personally like it this way.
Wrong, you're not really balancing the combat if you have to pause more?! Balanced combat in games like these are pre cast ex. Aura of Faith, iron skin ect. start a fight, then be able to actually see your characters gets some hits in without have to pause as soon as the first couple swings by the enemies deals damage. Especially in this game were there are no "cosmetic swings" damage even if its minor Is going to force you to pause and cast spells/buffs in response, instead of how it should be, just casting the buffs before hand.
This becomes even more evident if you can make a group that really starts to wreck the enemies in the later stages of the game. In previous games like this, Icewind Dale for instance, it was a reward when I finally had a group that could pre cast buffs that lasted for minutes then go on a spree through difficult mobs without having to pause. That is where I would definitely not want to have to start casting the same spells over and over after every fight.