Plus, it hurts in metrics used today, as well as in perception. Gary Patterson was a big fan of "win by a single point" but eventually figured out the need to run up the score (which got TCU into the Rose Bowl over Boise).
While resting is important, it is also very important to score TDs when the defense or special teams gives the offense a short field. Few teams can come back after being down 3 TDs, though it does happen.