I'm not sure if the description (-attack speed) applies to spellcasting as well, and I vaguely recall reading something about shields affecting spellcasting (for some reason, I recall it had to do with Speed, though I'm thinking it's more likely the Accuracy penalty of big shields)?
Edit: Raven's got the answer below.
As for my thoughts/advice, I find even on PoTD, as long as you have two tanks and position fights well at the beginning, it's rare that your back row gets hit. And in the exceptions, like fights with Shades and Shadows, I have found it more effective to focus fire on the enemy that has gotten to your back row, rather than playing defensively. I have tried the same Shade/Shadow fights with everyone wearing plate armor, ignoring the teleporting Shadows and trying to focus on the Shades. From personal experience, the DPS loss outweighs the benefit. Even with good heals, your back row eventually dies from loss of Health, not Endurance, and more Shadows spawn in the meantime. I have beat these same fights a lot more cleanly by focus firing each enemy that gets to my back line. Just when you think you need defense most on your back row, what you really need is good CC and DPS.
So I wouldn't take Weapon and Shield style or ever use shields personally on a non-tank character. If you can't find better talents, I suppose some defensive ones that don't depend on a shield could be alright, but I'd give preference to DPS. You can't use a wand with a shield anyway (say, for Rot Skulls). And yes, any shield bigger than the smallest kind will incur an accuracy penalty, and the small shield's defense is most definitely not worth giving up DPS for.
Some people may disagree, but this is from PoTD experience.
Edited by Nobear, 26 June 2015 - 12:46 PM.