Laws of averages apply to HP; I just change that law. I concede that a certain amount of Random is required, otherwise every fight is predetermined. I'm not advocating the loss of randomness, as much as I, personally, am advocating a general change from "utterly chaotic" to "controlled chaos" with an option for "unhealthy risk."
Point yielded: having HP randomly determined and not go your way can happen, and if that's the mechanic of your game, by all means, roll with it and see if your character changes. But I submit this is escapism, and I don't know any of my players who would be "happy" being gimped on the battlefield because their build was undone by a lousy die roll.
Combat has an inherent dischord to it. Randomization is necessary. Skills can go one way or another; especially under pressure. But would you, as a player, want to be sitting across from me, the DM, and be told that your straight 8s were your final rolls? I just don't think that would be fun for anyone.