Longer fights can lead to.

1. Monsters using more varied TP style attacks that have strategies to avoid and or minimize damage
2. Enfeebling effects have time to actually be usefull (Using slow on a monster that dies before he would even swing next is useless)
3. Incapacitation effects are actually usefull (Same thing as enfeebling mostly)
4. Gives players time to use positional tactics (Combos and whatnot are extremly hard to even try using on "Mooks" because the thing is either spinning like a top or dead before you even walk behind it.

I gave a few of a long list of reasons longer fights (I don't mean marathons for every fight) But a fight that can last a good 1-2 minutes per monster.

You say systems like enfeebling effects and combo system are not effected by fight length and you couldn't be anymore wrong. It takes time to cast a enfeeble and feel it's full effects. Being paralyzed for 5 seconds missing a single action (if your lucky to even have it proc once) is much different than being Paralyzed for 30 seconds and the monster missing several actions.

This works on the other end of the spectrum as well, nobody cares about debuffs like poison on themselves because shortly afterwards the monster is no longer a present threat and all attention is focused on you. Longer fights cause more scenarios that will test healers/tanks than short battles.