ald, you must have had a terrible DM. you see, a dm sets the scene, yes, a shield is automatically figured into your AC. also, the DM has the option of giving you a reflex throw to catch the arrow in the shield. but a good DM would give you the situation in combat, say an attack misses you but just barely, the DM could have said that you catch the arrow in your shield, enough arrows caught that way might slow you down a bit. removing the arrow could take you a move equivelant action, in which time you lose the ac from your shield, which presses you to keep up the attack, even though you\'re taking more arrows.
see ald, even with the rules, the DM is what makes the game. however, there\'s another secret, the DM has the power to take the rules, and throw them out the window. The DM uses the rules to his own discretion, it even says so right in the DMG