Possibly because those of us who'd agree that spanking as you define it is not a good thing, have been lectured in previous threads that our occasional use of, as you put it, a 'mild, open handed swat' is the same thing.
It's not an attempt to defend spanking as you've defined it, more a discussion as to whether such occasional 'swats' are, or are not, effective.
Can you define how you mean effective?
I don't think anyone really questions that animals, including humans, can have their behavior modified (at least temporarily) via pain. After all, I would say that's why animals evolved to have pain. Following the "if it hurts, don't do it anymore" standard is a pretty good survival mechanism. Sure, some things that hurt don't pose any real risk, but in the grand scheme of things pain is overwhelmingly correlated with things animals shouldn't do if they want to live (get cut, burned, broken, or bitten, eat toxic things, poke your eye, etc).
Perhaps you mean "overall there's a net benefit" to the occasional swat in that it merely modifies behavior in the direction best for the child and does nothing else negative or the negative things are outweighed.
What negative things might be associated with these occasional swats to toddlers? One that comes to mind is teaching the toddler and onlooking older kids that hitting is the preferred method of getting people to do what you want. Did you have multiple kids? If so, how did you explain to them that it was a good thing for you to hit Toddler but it was not ok for them to hit Toddler? Or did you allow your older kid(s) to hit Toddler? I've seen non-familial kids using Occasional Swat and other physical force on toddlers, much to the dismay and anger of Toddler's parents.
I think someone in this thread mentioned keeping a toddler away from an animal that might harm them. Let's say you left Toddler in the care of another adult. That adult chose not to move the animal out of reach of Toddler. Adult didn't feel like repeatedly getting up and verbally addressing Toddler while physically redirecting Toddler, so Adult decided to use up one his allotted Occasional Swats.
Is this acceptable to you as Parent? What if instead of Adult it was Teen Cousin? Teen babysitter? Daycare Worker? Tween Sister? Eight year old Brother? Neighbor Kid?
If all of these people can do it, then fine. If not, then I would infer that there's something negative associated with Occasional Swat that is offset by the Swatter being the parent/guardian. What is that negative and how is it offset by being a parent?
I've heard people justify Occasional Swat by saying what they were doing just wasn't working, like with keeping Toddler away from Animal. How does this flowchart work?
Moving Animal out of reach of Toddler is not an option.
Physically getting off my ass and redirecting Toddler for the 50th time is not an option (why?).
Getting the assistance of others to help me in redirecting Toddler is not an option.
Moving Toddler away from Animal (as in get up and go somewhere else for a while, including leaving) is not an option.
Therefore, inflicting pain is the only option left. Of course, it's not 100% reliable, so I still have to keep an eye on Toddler to keep her safely away from Animal. What if she does it again? Do I use up another of my allotted Occasional Swats? How many times do I get to swat her?
Do I just keep on swatting her? Most people tend to say no, I don't. Instead I am expected to exercise one of the options that I already said weren't options to justify the first Occasional Swat.
I know what's coming next: The world's not going to end because I swatted my toddler on the butt. Nobody said it would because obviously it hasn't. But don't think for a moment that some kids haven't been totally flummoxed at getting in trouble for using mild violence to modify the behavior of smaller kids after seeing parents all around them using Occasional swats when the power differential is even greater.
Me? I'd much rather be able to say to me kid, "Do I hit you? Should I start now? You seem to think it's a good way of handling things, so should I do the same thing with you? No? Well, why not? How come it's not a good idea?" And therein comes the empathy lesson.