I think it is possible to spank without being abusive and it can be effective, but isn't necessarily. I think punitive forms of discipline, which includes spanking, is a far cry better than not disciplining children at all. But I think there are better ways, with better outcomes and they are not that hard to master.
Spanking tends to require escalation which can encourage it to get out of hand. What's the difference in how hard you have to spank a 6yo who has always been spanked and one that has never been spanked before you get their attention? I guarantee you that six year old that hasn't been spanked before will not need much to get his attention, but the one that has always been spanked will require considerably more than when he was two, because he has gotten more used to it and it has had to be escalated.
I agree with the previous poster who said that it teaches a kid that he is less powerful than others and that the answer to solving his problems is to hit those less powerful than you. It would be better for the parent to model conflict resolution at that time.
I also believe in Kohlberg's stages of moral development, which I recognize as being controversial and I do believe that corporal punishment tends to stick a kid at the lowest level where "right" and "wrong" are based on punishments and rewards rather than an intrinsic reason for doing or not doing. It might change a kid's behavior, but does it teach a kid anything about why that behavior is right or wrong?
Someone said that it is fine under age 3-6 cause they can't be reasoned with or use logic. First, even babies understand language. They understand way more than they can speak at any age. If you assume your child can't understand anything he can't verbalize, you are soo wrong. Babies can learn what NO means and they can learn what tone of voice means, and they can learn "put that back" and so on. They can learn it without being punished too. But I also think that babyproofing should be done. A child that young needs freedom to explore things for proper development, and learning not to explore his environment early on can hinder his drive to explore figuratively later, like with career decisions, solutions to problems, etc. We tend to keep the same patterns of behavior until someone changes them, so how do you plan on teaching an adult to explore more, take a few risks, think outside the box, if that behavior was strongly discouraged as a baby?
Second subpoint here is that the love and logic method of discipline does not require the CHILD to use logic anymore than spanking does. Less actually. It requires that of the parents. Parents use consequences that are either naturally or logically associated with the behavior because it is easier for that association to form. Both spanking and logical consequences use the exact same theory of classical conditioning, but some things are far more easily conditioned to a certain behavior than others. If you assume a child can learn that x leads to spanking, then it should not be a leap to understand that the same child can learn that x leads to related consequence. For example, banging a toy against the coffee table leads to spanking, or banging a toy against a coffee table leads to losing toy. Running out into the road while playing outside leads to spanking or leads to losing outside play time and you go inside.
but I prefer for kids to be told what to do than what not to do,(i.e. push your car on the floor, when the child has been pushing it on the coffee table) given two acceptable choices (sit on the couch or go out and jump on the trampoline vs. stop jumping on the couch) (not acceptable behavior or punishment), and when_________then___________ statements (when your room is clean, then I will consider letting you have a friend over.) These things work. I have used them with my kids (transformed their behavior and my temper) and I have used them with kids I have taught. It works pretty universally.