Originally posted by FissionChips:

Well ♥♥♥♥ me, all this time I thought parry bonus was a damage bonus in riposte, but it's just block armour multiplier? Urgh. Might as well just tower shield, meh.

Parrying staggers them and allows you to crit them, which the tower shields lack. The parry bonus multiplier however has nothing to do with crit damage.