The sword wasn't a primary weapon, for a fully plate armoured knight. It was generally a backup to a primary like a polearm. (A whack on the head from a poleaxe is still going to take someone out, if only from concussion.) It would work well on lesser armoured opponents and wasn't completely useless on a fully armoured opponent. By "half-swording" you could grab the blade halfway, making it possible to better control the tip to get into spaces in the armour. It also provided leverage for wrestling an armoured opponent to the ground, where a dagger or half-sworded sword could get at something vital.