I should preface this with saying that while my understanding of childhood development and educational psychology is likely a bit more rounded than that of the standard tftv poster, I am by no means an expert. It plays an important role in the field that I'm studying, though (I will be a certified K-12 teacher this time next year).
If you have gotten to a point as a parent where your relationship with your child is bad enough that you have to literally hurt them to get them to behave/do something your way/etc, the act of physical punishment is just a continuation of poor parenting, rather than the start of it.
Studies have shown that physical punishment can negatively affect your child psychologically.
Studies don't shown physical punishment to be consistently effective.
It's a good way to lose the trust of your younger child or the respect of your older child.
This gets into much more a much more opinionated area, but I currently believe that corporal punishment is the absolute worst thing your can do to a developing child. It really instills the notion that the only reason they should do the right thing is because of the fear of negative consequences to them, rather than the fact that it's just the right thing to do. This same thing can be applied to parents that I've heard in public talking about bad people going to hell (god bless the south), etc. That stuff will scare the crap out of your kid in the short term, and really damage them in the long term.
Providing direct, negative consequences and rewards is somewhat necessary at a younger age, but by the time your child is old enough to have an almost-conversation with you, the focus really needs to be on helping your child out of their egocentric phase where everything revolves around them. When they truly understand that others are just as complex and emotional beings as they are, they should be learning their "golden rule", etc.
It's not situational. It's not a "last resort". It's not proven to work. Don't hurt your damn kids. It's a shame this discussion still has to happen.