I don't get it. My understanding of toothpaste is that it provides a polish, a fine grit, to remove the plaque that builds up, and some flavoring to make the process more pleasant. If you believe the ads, you might kill some bacteria and get some whitening along the way.
So what is soap going to do, other than taste bad? I tried it and would much rather go with salt, baking soda, a prepared tooth powder, or my current option, a small vial of gel toothpaste.