Darvin: The problem with shields has more to do with party mechanics. What you really care about is the weakest link in the party durability-wise, and with d12 hit dice the Barbarian is practically assured to be "not it". Further investment in AC is nice to have, but explicitly secondary to your primary role within the party. For the barbarian that role is dealing copious sums of damage, which is why two handed weapons are almost universally preferred to shields.
Without other party members this calculus no longer applies. There's no longer the concept of a more fragile party member that needs protecting, and you're free to bolster your own AC at the expense of damage output.
In a 6 man party, yes, solo, I'm afraid this logic doesn't apply.
In IWD2, the enemy more often than not goes for the squishiest characters like mages. With 6 potential targets (up to 12 with summons) and a lower probability of being targeted by the AI, front liners can afford to forgo a shield.
A solo character or course is taking all the heat as as far as I can remember, the only chance a non caster has to use summons is the wolf charm you can bet for in the targos tavern. A solo character is probably better off with the shield for survivabilty and use the terrain to control enemy movement