I think this is just a fast-and-easy solution until they have a real and balanced solution in place. At least it makes sure that you aren't punished for killing the zombies faster.Regarding the blood moon horde night, is there a reason the horde is limited to a certain amount of zombies per night?
If zombies were endlessly replaced whenever they died (so that always 8 zombies are attacking per player) someone killing the zombies faster would have to kill a lot more zombies. Sure, that also means more xp at the moment, but then the best strategy would be to just slow down the zombies as much as possible and shoot only the better zombies until you have only lowest-level zombies alive and then (!!) keep those alive.