I agree that they move rarely, but is the value of any unit only in dealing damage? Do no other tactical thoughts come to mind? For example, holding out until the hero's next cast or shielding matrons from any enemies of size 2*2 ...

It's better to use scouts for shielding matrons. And a hydra might even block the matrons themselves. Just standing there and pressing defense with the goal of holding out until the hero's turn won't work either. Because you've committed your fool, and then you just bash them for free with high-initiative units until it's her turn to move. Plus, her stats are very weak. How can a hydra stand up to a Druid? And in general, this is a problem for all walking monsters. On such a battlefield against ATB they are simply outplayed easily.