it's all about burst damage. That's what dual wielding is all about. Theoretically speaking it will lower your time to kill by 50%. Not sure if you ever played the older fallout games, but bursting down your enemies before they could retaliate was very important. It's the same in any game, really. Dual wielding has it's niche as a low accuracy "nuke". Good for stealth character or whatever.
That's not even mentioning the obviously benefit of being able to reload with one gun, and fire with the other, cycling between the two for continuous fire.