Nature Structures

the hero can, in principle, deal more damage than a group of water elementals... slow and not very useful units...

And black dragons won't even flinch. Don't confuse apples and oranges. First, the hero has much more important things to do. Second, there might not be sand in the tower. Third, elementals are useful in the early stages of the game.

Slow... Well, who, tell me, carries them around?
Not very useful... no comments.