Before you decide you're going to use Shield as an immediate interrupt, do you get to know the opponent's attack roll? Because the DM usually knows the character's AC, and will simply say the goblin hits you for 8 points of damage. How would he decide when to use Shield? Should the DM be saying something like "the goblin barely manages to hit you" when the AC difference is less than 4? Should the DM be announcing what AC a creature hits every time it takes a swing?
Also its duration seems rather short. If a harpy hits the wizard right before his turn, and he decides to trigger Shield, the bonus could go away before anything else can attack him unless he intends to provoke some OA's.
Also its duration seems rather short. If a harpy hits the wizard right before his turn, and he decides to trigger Shield, the bonus could go away before anything else can attack him unless he intends to provoke some OA's.