A Smart Button is configurable by the user. If the skin
has a smart button, in the Config dialog you can choose from 4 plugins. Three of the 4 are standard button functions like rollup, lock on top and URL. The 4th one is quick launch menu which is really handy but for some reason doesn't work on the new WB4 alphas. You also must be a registered user to access the smart buttons.
All of the above may be why there aren't a lot of skins that use them...