Seems a bit awkward to me. You'd be limited to using just one hand to fight while you're using the other one to shield you. Being able to shield for 60 seconds or so and then use both hands in combat would be more useful.
On the other hand, if you can time the shield spell right so that it's only active when monsters are hitting you, that's more magicka efficient than a 60 second spell that cost a lot of magicka, where half the time you're not even being attacked....
BTW I'm very much looking forward to what cool spells modders will be able to create.
I really like the way Duke Patricks Combat Magic mod handles this - instead of encasing the PC in an invincible cocoon, a shield spell creates a visible force shield that you have to hold (draining magicka) and use directionally, in a strategic way. You can let go of the button, attack and raise the shield again.
If you use a shield spell in Skyrim in your right hand, however, you can still wield an attack spell in your left hand (or a weapon), even as you are holding down the right attack button for your shield spell.