I'm not saying there is a situation where the ONLY appropriate punishment is spanking. But if you try all of the other discipline methods (time outs, loss of privileges, emptying the room, etc) and they DON'T stop the poor behavior, it's possible spanking might.
One thing I can think where an immediate spank gets the point across, is you've taken your child to the park. You're heading to the car, the child gets away from you, and runs out to a busy road. Fortunately you catch them before they run out to the street. Do you punish them? How? Wait until you get home and put them in timeout? If we're talking a young child, I don't know that they'll register the punishment with the crime.
If you can point out where someone has said that, I'll agree with you.