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n00bdragon , I understand your point and will try to explain mine more clearly. You are saying that if the players REALLY want to know, they can figure it out. I agree. That is scenario #1.
Scenario #2 involves players (like those I used to play with and myself) who find it more fun to not be completely sure. When in "competitive" mode and trying to make our characters survive and succeed, we might be tempted to use such knowledge. However, we have realized that in general we enjoy ourselves more without doing this so it's a sort of gentlemen's agreement not to. Informing them that a monster is at half hp is because we agree that the characters would have some idea about how beaten up it is getting.
If it is hard baked into the rules that everyone is told monster hp so that hp thresholds work, then scenario #2 isn't going to be a very viable one. However, if it is not hard baked in, then those who enjoy more of a scenario #1 playstyle can still do that at there own table while those who enjoy scenario #2 get what they want to. i hope that explained my viewpoint better.