It really isn't necessary in this game. You are either attacking or not. There is no reason to need to block. It isn't like they have special attacks to try and block or anything like that. Yeah, there are the ranged poison attacks, but those are not exactly difficult to avoid as it is. Otherwise, all attacks are basically the same and rather than spending time blocking, you can just kill them. Blocking is great in other games, but I really don't see value in this one.
I'd agree with zztong that I wouldn't mind it being there, but don't miss it (and probably would never use it if it was there). As he said, once you know your weapon, you can avoid being hit pretty well. And if you are swarmed, standing there trying to block isn't going to work well.