It's not an effective punishment and has the possibility of causing your child lasting harm. This shit has been researched for decades but what do you know - people trust anecdotal evidence more than experts.
I think part of the reason why it's so hard to convince people to quit spanking is that people are very unconfident about admitting their parents might have raised them "wrong". You look at the reasonings of spanking proponents and one thing always repeats: I was spanked and it didn't do me any harm. That's an incredible defensive comment when you think about it, and very telling.