I think the new system will be great not only for balance, but for replayability... now there are reasons to go back and play a pure mage after your combat build because it will feel that much different, instead of just adding magicka, dropping heavy armor, and altering certain aspects of your game. Now barbarian that uses a two handed axe will feel different from a sword and board character, and a mage will feel way different than either of those two. You're not necessarily limited in your options for a single playthrough, you're just encouraged to stick to a type or combat.
Personally I felt when I played oblivion I always had to use magic because there was a button devoted solely to magic. It felt I was shorting my character by not throwing a fireball or two. Now if I play a dual wielding berserker, I won't feel so bad about not using magic... and my mage type won't look so dumb running around without a weapon cuz I can have a spell in each hand :clap:
EDIT: I just realized how unbelievably off topic I got with that.... sorry lol. My post still stands.
And to answer the question... if you go sword and board or dual wield and you wanna use destruction or something, then I think the best option would be to unequip your weapon and keep the shield for defense( or just one weapon for dual wielders lol). It's a give and take, and being able to attack, defend, and cast isn't fair to pure fighters or mages, so two hand two options makes the most sense, in my eyes !t least