If you're a game designer and you have to choose between balance and common sense in your game, then you know you've done a crappy job.
What an absurd statement, plenty of games make concession's in realism for the sake of gameplay. In fact, almost all games do.
Furthermore, JustinOther, the idea that the player is defenceless while dual-wielding is kind of a misrepresentation. You can move, can you not? Dodging and out-stepping your opponent are probably going to factor quite strongly into the dual-wield playstyle. You're only a sitting duck if you're sitting still.
Dual-wielding allows for increased offensive opportunities, it's a very simple and clear design and balance decision to give it reduced defensive capabiltiies to compensate.
Ideally, you would be able to parry or dual-wield with two swords, but with a lessened effectiveness, but then you change wielding a single weapon into a functionally identical experience to dual-wielding, but simply mathematically different, which is a flawed design decision to make in an Action/RPG.