You can't block, so you take more damage, and thus get staggered all the time and therefore wont be attacking more than you would with a one hander and shield.
You can't block, so you can't stagger the enemy, and thus the enemy will block more, and thus take less damage and stagger you more, meaning you don't actually get more attacks off than with a one hander alone.
If you aren't using dual wielding sinply as a secondary backup to a ranged ability, and you want to be melee oriented, you will have no crowd controling effects such as knockdowns or staggering from shield bashing, blocking, ect, and thus you will take more damage and likely get hit more and thus do less damage because you are staggering.
You take more damage, so you have to dump points in heavy armor, but you don't want heavy armor because you are obviously making an "agile" character if you are dual wielding, and thus you dump your points into health.
Why would anyone dual wield again?
I realize that people may be concerned about how this would function from a player-interface standpoint. What controls would you use to block when dual wielding if you already use RT and LT to control your hands?
There is already a system in place to support this. When dual wielding spells, if you pull the RT and LT simultaneously, it creates a more powerful spell. Why not expand this specific control to include parrying when using certain setups? This could work when following the following ruleset.
When Pulling both triggers at the exact same time and using one of the below weapon combinations, your character will do a parry. A parry will deflect less damage than blocking with a shield, and cannot deflect ranged attacks or spells, only melee attacks. Enemies will not stagger when their attacks are parried, but the damage of the attack will be reduced. A parry must be timed right to deflect damage. You cannot parry if you are only using daggers and have no one handed or two handed weapon equipped.
1h Weapon + 1h Weapon
1h Weapon + Staff
1h Weapon + Spell
1h Weapon
2h Weapon