Secondary skills in Heroes 3.

Why? It's just unnecessary clutter... walking units are generally slower than flying units... sure, you can break through a wall... but you still won't reach it in one turn...

You forgot an important fact: if you have a controllable ballista (with a skill, that is), then (assuming your opponent doesn't have artillery, which gives control over towers) the ballista moves first, and during its turn, you can cast a spell first! And this first cast can be critical!

All of this is, of course, relevant if you are besieging a castle.