If you don't have weapon mastery, using two weapons:
- Changes multiple parry penalties from -0/-4/-8/etc to -0/-2/-4/-6/etc.
- Allows making what is effectively rapid strike plus deceptive attack 1 at a penalty of -4/-8 instead of -8/-8
- If you have ambidexterity, the parry penalties become -0/-0/-4/-4/etc, and the attack penalty becomes -4/-4.
This is competitive in value with the benefits of a cloak or small shield; a medium shield will usually be superior but is much heavier. If you do have weapon master, I can't see a good reason to dual wield, it should probably reduce DWA penalties the same way it reduces rapid strike penalties.