Would be pretty cool to become an impenetreble fortress by duel wielding shields, then shield bashing your way through dungions. I mean, the infrastructure is there so why not allow such a thing? Again even if it's not practical, it would be pretty hilarious and give the game much more creative freedom in what you can do
Similar to how you could equip two different types of pauldrons in morrowid. It's just nice to have that kind of option or depth avalable should someone choose to do a goofy thing like that, which really reinforces the belivable, open ended non-linear world feel 8)