I also wondered if attacking someone's armor (how to target the armor - no idea) would be a great tactic since the penalty for wearing broken armor is high, it's too difficult to remove, and breaking items is easy.
Overall, I think the issue is wrapping our heads around how shields can give DR 1/turn (don't forget - if you're fighting a horde of small things, you can only block damage once per turn) but the cost is that you're going to break the shield in one or at most two uses of this. Even an adamantine, legendary shield can get broken pretty easily by a level 1 character's lucky crit or two. The problem isn't necessarily the tactic - it's that no one can believe that's how it works because it makes blocking with shields so useless. Then again, you have to waste an action EVERY TURN to gain any benefit from a shield - even when it's worn, and even though you always take the penalty from wearing it - so it's sort of par for the course.
As written, it seems like shields just negate their hardness in damage - so 8 damage would mean you take 3 and the shield blocks 5 and takes a dent. But the confusing part is the example (p.175) about taking 10 damage and the shield takes 2 dents. Does the shield negate more than 5 damage? Why would the 10 damage attack break the shield when the 8 damage attack didn't? Or is it just talking about the shield as an item, not how it is used in Shield Block? Personally I think using Shield Block means the 10 damage attack is reduced by 5 and the shield takes 1 dent - i.e. you can never take more than 1 dent using Shield Block.
Either way, shields seem extremely bad right now: massive action waste + too easy to break. Hopefully this gets addressed soon.