Usually the murder-stroke would only be used in half-swording during armored combat.As such, it is not common that either you or your opponent would be relying on a slash and hack type of assault.You would be trying to wrestle one-another to the ground so that you could run a dagger through the others visor or throat.With that in mind,there would be little need to parry a cut from your opponent while armored as plate is largely invulnerable to such cutting attacks.Your primary concern would be the afore-mentioned takedown or simply running him through via a strong reinforced thrust.Also,the gauntlets worn while armored would protect your hands from most incidental contact.
Just to be thorough, while I don't believe the murder-stroke is a valid technique against an un-armored foe(why give him the hilt when the blade will do?)there are many half-swording techniques that ARE applicable to un-armored combat that are demonstrated in the manuals and it is good to practice all of these as part of your regular training and incorporate them into your sparring bouts.
