it's better to have 70 vampires than a dozen useless piles of bones/ectoplasm, isn't it?
I often don't deploy all my troops onto the battlefield. I deploy one or two stacks – the ones that are most suitable for the given situation – so as not to waste the rest. For example, against a horde of demons. Why? Because with them, you can eliminate the demons without losses. If you deploy others, they will attack them. Is that a dilemma? That's precisely why I've always considered Heroes 5 to be the best tactical game in the entire series. Not deploying troops is simply a great feature. Initiative is an excellent innovation. And so on.