Plate protect against low base damage weapon, cut weapons, cut projectiles and early attack animations. But as soon as you have pierce/blunt and you aim your swing nicely, plate only cover a fraction of the damage or not at all for the most powerful weapons in the game (mauls and big axes, throwing lances, arbalests, ballistas). I think it is correctly balanced so that only the weak can not do any damage against plate, the average can still hope and the strongest can kill.
What is really problematic with plate, is the upkeep. Most of the expensive piece of equipment in cRPG comes with a terrible weight or speed, thus poor manoeuvrability: something that the 5% raw value for repair cost doesnt take note of.
off-topic:
This is the case for horses, armours, and a lot of big-punch weapons that are often hard to master.
For example : choosing the mamluk horse instead of the war horse, most of the time you will come short against a rider that chose the war horse, and you will repair more. Same thing for great sword versus longsword, same for gothic plate versus kuyak.
Repair cost should use the speed/weight of the item to decrease/increase % value repair cost accordingly, so that strange playstyles may play in peace and get killed with a smile, as much as every other player. If someone needs a nerf to go along with it, you can increase the price of such items by 10, but upkeep isnt working.