Ok, has ANYONE ever had a group who encountered a hydra and actually tried to cut all its heads off and burn the stumps instead of just attacking the body? (or better yet, attacking with only ranged attacks since it had none of its own.) I just don't see any reason why you would try to sunder all it's heads as the other way is way easier.
Only in a 5e game, but that one both conveniently gave us a flaming longsword immediately prior and apparently the way hydras work in 5e is that for every X damage taken in a single round they lose a head, no need for separate targeting.
This has always been an issue with hydras in D&D(at least until 5e apparently). Even when players are unaware of what a hydra is they have no reason to consider targeting the heads since you can't normally target body parts like that.
IMO, the hydra's heads should be able to use the feats "bodyguard" and "in harm's way" with it's heads. In this way it actually provides cover for it's self and it makes it much harder (though not impossible) to actually target the thing's body.
Yeah, it's just a bad interaction of system mechanics. Basically nothing else in the game allows you to target specific body parts, so if players are familiar with the mechanics of the system, they're still likely to just attack normally.
It doesn't even really benefit you much to attempt to target the heads and burn the stumps vs normal HP attrition.
I remember once in D&D 3.0, someone used Great Cleave to sever all of a hydra's heads in one blow. I have a feeling the game's designers wouldn't have allowed that. It's probably not what they intended.