Increasing the number of opponents will significantly increase the waiting time of the player, thus significantly increasing the time of the fight.
Fighting a huge horde of enemies is really not a good idea in a turn-based game.
Fights like a goblin camp are already long now and if you increase the number of enemies the situation will be even worse.
You just don't start with arguments that you can use AOE then because this argument sucks and only works if you have the initiative and / or enough sensible AOE spells (which you still need to hit).
