I think you're completely mischaracterizing corporal punishment. It's not a last resort. It's not like a parent tries yelling at a kid, then grounding him, then sending him to bed without supper, then finally slapping him out of frustration. Corporal punishment is most effective when implemented immediately, since it creates a stronger association between the pain and the unwanted behavior. If you delay hitting a kid, you risk them repeating the behavior much more readily. Also, when you start hitting a kid out of frustration, you've completely lost control of the situation as a parent. That's not the case at all when you do it immediately.
As I said higher up, my parents hit me so that they could threaten to hit me later. They made a couple dozen spanking last years. They never hit me after the age of 10 or so, because by then I was rational enough for them to talk to me. But when I was younger and lacked higher-level reasoning, hitting me was a great tool. It's much easier for a kid to understand that doing something bad will result in them getting hurt than it is for them to understand the complicated social mechanisms which their behavior can cause.
Now obviously some parents go too far and beat the crap out of their kids. Some punish kids who haven't done anything wrong. Some get a sick enjoyment out of hurting their kids. Irresponsible people mess up a lot of things. But it's not like this new generation in which corporal punishment is shunned is any better. Kids are still being incorrectly punished and growing up to be pieces of crap. I'd be very interested in a study that explored whether responsible use of corporal punishment really has a negative impact on children (controlling for socioeconomic factors, of course).