I think many parents strike their children out of a lack of being able to relive their stress somewhere else. When you keep frustration bottled inside you about other things, you tend to take it out on other people, more often people that you care about. Also, many adults who hit their kids where probably hit or whipped as a child (My mother knew kids who were whipped at her age.)

Getting a smack every once in a while is ok, but only if you do something really bad or stupid on purpose (Like key-scratching the symbol "8=>" into your teacher's car or punching a hole in your wall because you were forced to eat healthy crap food or something). Anything on accident should only recieve verbal discipline or warning or a talk-to (Which was what I recieved when I did something on accident as a kid).

The difference among people with this issue is where they draw the line between physical discipline and child abuse.