there's one thing I do not understand in this change (or maybe I have missed this): with the new rules and more uses, defensive items can be a block or an armor.
if that wasn't changed then following rules are in use: if a block is overcome it winks out. if a armor is overcome it still stays in place (to the end of the spell's duration).
in the new use-based system I see no "need" for the armor effect. take the "block 8 or armor 4" example. if I am hit, it uses a charge and I can decide to block with 8 or have armor with 4. since the block would allow me to entire avoid the damage, most people would take the block. or can I decide, after the charge is triggered that for the current encounter I have block 4 for 4 excanges or an armor of 2 for 2 exchanges? I mean could you divide the power of the block/armor on duration after triggering? (then armor would still make sense.)
So, first off, we're definitely coding in that you can't stack things like armor. So that plays into this thinking. If you had a shield bracelet too, say, and you threw up a Armor:3 shield and something blasted through it, and you're wearing a coat that's Armor:2, that additional protection isn't going to pay off. And it's already clear that it's only the best block action you have that applies, they don't stack either. But being able to frame each of these effects differently means you can get the best of both. You throw up a block effect with your shield (or your coat) and then follow with an armor effect from your coat (or your shield), and you're golden. That's really the only way to get good layered protection going: block, then armor.
Going further, yeah, you could define an Armor effect -- like any evocation effect -- with a few shifts of the casting strength put towards duration. So let's say you've got a Strength 4 armored whatsit. That could be Armor:2 for one exchange (a 4 shift spell effect); or it could be Armor:1 for 3 exchanges (2 shifts for Armor:1, 2 shifts for +2 exchanges of duration). And as you note, the armor won't collapse if overcome, like a block would regardless of the duration put on it.
So ultimately the way I'd roll: I'm Harry. My coat gives me a solid Block action when I don't have other protections up -- someone drives by, I'm unaware, they open fire, yay coat! If they don't roll to meet or beat its block strength, no damage at all. If they do, well the attack connects but at least it's mitigated a bit over my completely flatfooted surprised Mediocre defense.
Now I'm in the scrum: I'm calling up a shield effect because, whoa doggies, they're coming at me with everything they've got. Shield goes up, maybe at max power, I'm defending with a Superb block. Disaster! They hit me with an Fantastic (+7) attack. 1 shift is getting through and it's Weapon:3 -- I'm going to take 4 stress. But I've got an armored coat under there. I use it to get me the extra protection of (say) Armor:2, and it's only 2 stress. Good news, because I have a feeling I'm gonna need that #4 physical stress box before the fight's over.