Coding-wise, it's not difficult at all to store a simple piece of info on each Hydra Head that refers to the creature space occupied by the Hydra body it came from. If a Hydra is in any way removed from the field, all active creatures are checked to see if they are Hydra Heads and point to a "mother space" that doesn't contain a Hydra.
So I don't think the limit is remotely necessary, though it's not problematic either, since we already can't have two Chimeras even with PU.