Well i mean glancing and that stuff is just what plate does, its the same if you are using a str build. Plate leads to effortless valours, you get what you pay for. But its not like it makes you an immortal super warrior just because you combine it with agi, it just does what plate does. You can do the same with a reversed build just in a different way.
I dont know if you have even tried using plate as an agiwhore, but the movement speed you lose is pretty huge. I'm at around 25weight now 72BA 16/27, i move at about the same speed as a balance build. I can tank 3-5 good hits, i deal decent damage, 3 or 4 hits with GLA to kill medium armor. I can sometimes escape situations where a str build couldnt, but i also cant destroy people with 1 hit as a str build could. Im a slightly tankier balance build with less damage.