| >>Paladins don't need to be the only plate users.|
They aren't. You must be doing it incorrectly, because I see many non-Paladins using plate. My WM uses Plate.. Paladins do get a bonus to using Plate armor as one of their extremely, extremely few Guild perks.
>>I think the issue, in general, is that the visible amount of benefit you get from armor ranks stops around 200 ranks,
You are wrong. Armor turns into tissue paper if you are fighting something with more attack skill than your armor skill. Armor is a defense, just like parry, evasion and shield. Plate is less affected by this penalty, effectively bonusing lower-skilled players that decide to use it.
>>hindrance that comes with plate armor is too huge,
Hard to say. One defensive buff should generally overcome the hindrance penalty. If we make the penalty too small then why use anything but Plate? Brigandine exists as kind of a go-between, but nobody wants to be second best.