I've noticed that if you block with your shield after an enemies spell has connected with you, it doesn't avert damage. So for example, a dragon begins shooting fire at you, if you hold up your shield a second after the fire hits you, it doesn't defend from the rest of the spell. Wouldn't anyone else prefer it if you could pull up your shield in the middle of the spell to deflect the rest of the incoming damage?
I should also make it clear that I have the elemental protection perk.