[Baalz]

Hydras most certainly have a niche role. 250 gold is a lot if you plan on losing them while raiding PD, but 500 gold might be a cheap price to pay for major confrontations if it, say completely blunts 2000 gold worth of elephants, or holds a large uber sacred force for an extra couple critical turn (not unreasonable if combined with skellispam) while the poison damage mounts up. The gold cost is particularly relative when you consider that its not uncommon to be resource constrained as a nasty army is threatening your capital and you need to recruit troops fast.

Of course, as I mentioned I don't generally use them too much with Sauromatia, but I can certainly imagine situations where I would.