niteshade6 said:
I suppose a tower shield might protect you from a line breath weapon, but a cone would just flow around the shield. It might still give you some cover, but certainly not 9/10.
That depends on weather or not you consider a breath weapon to be an Emenation...
My Dwarven Cleric does this very tactic. Keeps him safe in a fight, and prevents most AoOs when it's time to heal in combat.
My two coppers would be,
3/4's for bows, providing the shield had support.
9/10's for a crossbow.
For crossbows, at least light ones, are small weapons, and can be used one handed just fine. The penalty comes from using two at once. Even barring this, I would think that you could steady it on the edge of the shield and take no penalty.
Loading and firing each round might be a problem, but you could always say you are using a foot stirrup.
Good point on the moving around the shield. I'd also add that cover might be relative to the field. If your surrounded, your taking cover vs the opponent of your choice. The rest fall some where in between. Like the Dodge feat.
As far as the AC bonus for +1 and more, I never would have thought that. The bonus HP would be nice, make it a +1 for the incresed shelf life, then stack on Fortification and Invulnerability!

I could see maybe giving you an additional cover bonus. A +1 shield gives you 3/4 when you only take 1/2, making you better in a fight by not taking the same penalties that you give. Not saying that it's balanced, just an idea that is more in line with how it already works.
If it's supposed to give you an AC bonus as normal, Full-Plate + Tower Shield just got scarier than it already was.