I can only speak from my own experiences, but spanking a child, regardless of whether it stops the behavior, still teaches the child that it is OK to hit people to get what they want.
Perhaps a spanking would be necessary in extreme situations...perhaps there are other alternatives.
So for, for us, the time out and reward system works really well for us. It's not clockwork, but for the most part, we've never had problems with it, our our little one is very well behaved.
I walked in on her trying to stick something in the wall outlet (we have those covers on them usually, I don't remember why this one was out) and I whipped her up off the ground so fast that the shock scared her, and then I was very stern (probably close to yelling) with her that it is very naughty play with those.
She was in tears and saying sorry so fast that I didn't even think about using a time out. She's never been near the outlets since.
I guess I'll see what happens if one of my kid's ever realizes that a time-out doesn't really mean anything...