Generally violence begets violence.
However, I've used generally very consciously, as I think there are circumstances when spanking, while still maybe not the best choice, is justifiable -- or at the very least, understandable. And I don't think parents who occasionally spank their kids are bad parents or should necessarily be chastised. I don't think as a regular recourse it is particularly effective, but, as Miss Chicken pointed out, sometimes children do things that are dangerous and are too young to be reasoned with. My mom hit me when I ran out in the road at age 2. She hit me twice growing up, and when your mother hits you only twice in your whole childhood, those instances stand out.
I think this generally sums it up. I'm opposed to smacking or spanking, and won't be using it on my own children, but there's a big difference between an occasional smack intended to emphasise dangerous or truly unacceptable behaviour in the very young and an actual policy of correcting behaviour through violence. They're not really comparable. Which is why, though I'm against physical punishment and would strongly urge others not to use it at all, I don't think smacking should be illegal
, and I wouldn't say a parent who smacks occasionally is abusive. I disagree with their perspective, but that's rather irrelevant. And as you say, parents who smack in exceptional circumstances have understandable reasons for doing so.