I have to confess, I have no idea what you're driving at here. You seem to be saying that someone who is a trained and experience professional with hundreds of hours of drill and practice has no reason to expect better performance at his chosen field than a rank novice would.
And if that is what you are saying, then I can only say "Fine. As long as my Fighter is then just as good at throwing fireballs as the wizard. Since training and practice are not factors."
In reality a trained and experienced fighter is going to be stronger, faster, with better reflexes, better situational awareness. He is going to not be paralyzed by fear, not frozen by uncertainty. He is going to see his opponents and be able to make quick judgements about their capability. And yes, he is going to be tougher. He is going to know a cut on the arm is painfull but not deadly, and he is going to not panic, or be distracted by his pain. I once heard a friend of mine (IRL, not game) say "The first time I got shot I knew I was dying. By the 7th time it just made me mad." Sounds kinda like HP to me.
1" of steel in your throat will kill you, regardless of who swung it. But a Roman Legionaire is a lot more likely to put that steel in your throat in a single swift and certain stroke than a panicked accountant. And he'll be blocking your shot and sizing up the next opponent while doing it.