If you look deep enough, I am sure you would find a ton of things that we calculate and check for and send updates on regularly that in a perfect world, we wouldn't need to. The problem is that it isn't a perfect world. Another problem is that a large number of hacks would be possible if we didn't at least try to keep the client in check and do everything server-side that we do currently. In the case of HP updates, they are needed for both clients as well as for NPCs. The client does not ignore them, though it might try to do it's own calculations on it's own, it will still ultimately do what the server says. Also, you need to take into consideration that not all servers try to emulate Live exactly, which means that the server might not calculate stuff exactly the same as the client. We have a rule that allows server admins to decide when Out of Combat Regen starts happening and what rate it recovers at. So, without sending HP Updates, the client would never be in sync with the server in that particular case.
I am sure there are plenty of other good reasons, but I think that one alone should answer your question.
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