I find it so hilarious that people are debating the realism of dual-wielding in a game full of unrealistic abilities.
It's a feature for those close-combat moments where you just want to send as much lead flying as possible.
It's got nothing to do with practicality, it's the satisfaction it provides.
Why wouldn't you use an assault rifle or a machine gun if you wanted to send as much lead flying as possible? It's like telling me I have to shoot something IRL, accuracy is of utmost importance then giving me a choice between a handgun or a rifle. I'm never gonna pick the handgun.
Of course, I get the feeling a lack of dual wield has nothing to do with the realism factor. It seems more likely they put together a set of features they might consider and in the design process decided some of them weren't going to make it. DW didn't make the cut (or it did? I dunno... you get the idea).