I don't really think that PolyMorph is that great of a spell. At least when I've been playing... I mean, really... what kind of units does a player usually have when they have polymorph? Sorceress' and priests. Of course that's a tough combination, but this thread is about countering dryads, which can't be poly morphed, and niether can heros. So I guess that means my huntresses get morphed and then the attack moves towards the dryads, I'd take a dryad over a sorceress any day, because usually I'd go for the priest first, so they can't heal the sorceress' and then hope that my huntress' survive long enough to morph back and really start mauling.
I don't think the game is strictly about the power of each unit, but it's the how you management of your own units... such as how you kill individual units with your units. It's obvious that spell casters aren't going to be as strong as melee units, or even ranged units. They're meant to cast spells, not fight. But to me sorceress and priests is a bad combo, I'd be more willing to take my chances with knights and priests... sorceress' don't seem all too powerfull, outside of polymorph. Once you've got the unit morphed, that's great, now it takes you forever to kill it because they're attack isn't powerfull, it's just a delay to me.
But again, I play elves, and never play humans. Some human players are better than others.