True, no summoned creature can take on a fully developed demigod hero (at least not a ranged one) but sometimes I prefer summoning a couple of creatures for the same cost as creating weapons/... for a hero.
Especially if your opponent starts casting cracks call I'd rather see a behemoth disappear than my precious mage/archer with 6000 worth of magical items
It's just a different tactic just like there are people who stick to berserkers or paladins all the way.