I've suffered two metacarpal fractures in my time punching things. One was pretty serious, the other less so. While I agree that punching does have its place and is sometimes the only option to you have in a given situation, I can't recommend it as the best way to do things retgarding hitting the head. I should point out, too, that both of my fractures were during training, not during a fight, so there was no adrenline rush, and my experience certainly wasn't anything like Ninor's. I did all of the karate-style knuckle conditioning, too, withthe makiwara and doing knuckle-pushups.
The problem with the first incident was that it was a structural misalignment caused by me doing some experimentation during training, in that I was mixing a katate techique for punching with a Wing Chun technique, and it was a bad combo. The karate style uses the heads of the first two metacarpals for punching with a strong wrist alignment. The Wing Chun Chung Choy punch uses the smallest two metacarpals with a bullwhip type delivery and no hip movement. When I added the powerful karate hip twist in with the Wing Chung alignment...ow, it broke, and it broke good. The second time was an accident and only a minor fracture...however, both hurt a lot and put me in a cast for 6 weeks and have left minor residual effects.
I think the problem is two-fold. One is the ever-mentioned muscle memory. You do what you train to do when you're not thinking. The other is that people are--men in particular--by social conditioning, head-hunters.
I believe the first one is the real problem, which is exacerbated by the second. The head is simply harder than your hands will ever be. You can hit it okay under the right conditions, and get away with it, but in my opinion if you do this a lot a broken hand is a matter of when, not if. In the swirl of a slugfest, getting the proper alignment and execution may be problematical. I think conditioning an auto-response of punching the head into your muscle memory is setting yourself up for a self-imposed injury. A target of opportunity, like a heart shot or a bladder shot, may indeed be an appropriate target for a punch. But most people fire them off automatically and almost always aim for the head. I think that is a bad idea, strategy-wise. At least my own experience seems to back that up. One of the objectives of delivering any blow is to hurt the enemy, not yourself.
The most important aspect of this is that punching usually isn't necessary. There are other ways to hit that are just as powerful and risk your hands far less. The problem is our social conditioning: we're all taught that punching is how you fight like a man. That's the way Pa did it. That's the way movie stars do it. That's the way boxers do it. This bleeds into your psyche over time. Then you lock that into your muscle memory and as soon as a fight's on, that's what you do without thinking. Pow! Potential broken knuckle or wrist.
I'm not so sure that punching becoming a lost art is a bad thing in terms of self-defense. It might well be so in TMA, where it is the core of some arts. I do know I can hit much harder with a palm or elbow than I can ever hit with a punch...at least at power levels that would seriously hurt my fist. It may or may not deliever as much damage potential to my target, but it delivers an equivalent amount with far less risk to me. But I also know that when the scatalogy hits the oscillating turbine, you will do exactly what you trained to do if the need is fast and furious, because you won't have time to think. Because of that. I think it's best to load your auto-responses with things that don't poentially hurt yourself if you can. I've really de-emphasized punching in my own toolkit for that reason. But that's me, and we each have to find our own way along the path.
Ed