Hydralisks were so much more useful in Brood War, where they were so much faster, on a lower tier, cheaper and could transition into Lurkers.
They're now slow, hard to get to, and the Lurker's role has been taken over by the Baneling. Great if you have them around for anti-air, but unneccesary and the Mutas and Corrupters you probably have at that time are more effective.