ok...rather than engaging in the whole "i smack my kids and it never harmed me..." OR "I never EVER smack my kids as it teaches them violence is good..." etc, here is a thought on the subject;
I was in the kitchen this morning making breakfast for dd3 and she was climbing to stand at something close to the cooker. I told her "no" and "off" (which she does understand), and she did not listen. I removed her from the area.
She went back towards the same place, and i warned her away again (this time before she started climbing), and then removed her again, giving her a toy to distract her.
The THIRD time i didn't warn her, i just removed her, but accidentally knocked her on the oven (which was not on btw). After a cuddle and my feeling guilty, i put her back on the floor and she didn't go back.
I find this has happened in the past...she has pulled herself up on chairs at the dining table and hurt her head on the table, so she has been more careful since. She learned (as all babies do) not to throw herself backwards from a sitting position because she'd hurt her head on the floor. Also these experiences generally toughen our kids up so they do not always cry when they land on their bum after attempting to scale the settee!
Does pain teach? Yes, undeniably. Should we inflict pain when it isn't necessary? No.