Actually Brian Gardner removed the one-fifth part from the css….not Wes (or Appfinite). I remember him mentioning why on Twitter not too long ago, and it was due to measurements or something.
You can look at the Genesis Sample theme to see that it’s no longer there – http://www.genesisframework.com/columns/ ….and the reason its not in your theme is because Wes develops everything from scratch on top of the Genesis Sample theme.
But of course you can always add it if you want, you’ll just have to figure out how you’re going to add the measurements.