And regular units don't really offer anything good... Do they have a constant retaliatory attack, or not? - I forgot. And the "Strike from Heaven" spell is very effective against archers, and besides, a griffin isn't a target that's usually attacked at the very beginning of a battle, meaning that even if it successfully attacks the archers, it flies away (even if not very far), and the opponent can spend their attack on someone else. And even if they attack the griffins, it's not a big deal; they'll fly up again and either prevent the archers from shooting or give them a good smack. Which, in my opinion, provides good opportunities for infantry and cavalry.