I really like that rule and it jives nicely with my real life experience. I've fought plenty with two weapons (sword and dagger, mostly). Using two weapons is something that gives you a big benefit primarily via an increase in defense (and by increasing your offensive options after having made the parry, ie. as you make the parry you have a free hand with an unengaged weapon available to do it's thing right then while the opponent - who is in range because of having made the attack - has nothing