There are two troubles with one handed plus shield (it works quite well in act 1 as you correctly note but less well later in the game).
One - the shield slows you down so it isn't exactly free defense. Not a huge deal but that negative to accuracy, speed and initiative IMO outweighs the benefit of another item that you can have charms in.
Two - you will face more and more enemies that attack in non-physical ways so the shield blocking gets less and less useful. Nice to have but as noted in the prior post some of the magic damage they do is serious and you need to take them out as quickly as possible - doing 2H damage is nice.
1H with shield works but I agree that 2H is better.
