RTF is a global modification that replaces all towns in the game.

What's the point of sending a single flying unit behind the walls? To be slaughtered? + Shamans have the "earthquake" spell.

This is already a matter of battle tactics, and everyone has their own. A complete absence of flying units will create problems, including during battles in arenas without fortifications - obstacles, etc. Earthquake only partially solves the problem. Of course, it's up to you, but balance is a fragile thing; in my opinion, it would be desirable for each city to have at least one flying unit and one ranged attacker.