So, I feel obligated to point out that China is a very big place with a lot of martial arts styles, and the exclusively dual-weilded butterfly swords are way more common than the shield in my experiance, and that's not even touching on all the other dual-weilded weapons in which I have none.

As for why you'd dual weild instead of using a shield, well, if you're excluding all the noncombat things like concealability, manouverability and redundancy (and you probably shouldn't - a shield's efficiency in combat is cumbersome in another), a pair of short weapons is remarkably effective against spears and poles, which are very common weapons. An offhand smallblade gives you a level of control over your opponent's weapons that a shield does not.

Granted, this does not make dual-weilding automatically superior or even practical in many cases, but remember that the shield is the tool of a soldier and is best employed in ranks, and a D&D group is generally not those things. Rule of cool may be a very big draw for a lot of people, but there wouldn't be dual-weild styles at all anywhere ever if it wasn't a practical thing in some capacity.

~Silv'