Just wanted to clarify I know realistically anyone can wear plate armor. Real Knights could run around and even do cartwheels in well made armor, it's a myth that they had to be lowered onto their big horses on cranes (like in some old movies and books mocking the middle ages). In the same way, Longswords and crossbows are treated in this game like one handed weapons when they really required two. And we all know from LOTR (the inspiration for all of these fantasy games), that Wizards can use swords. These are game balancing mechanics, in the end.
With my Paladin character I interpreted it more like the Plate armor is restrictive which accounts for the halved movement, but because he is trained with it, he isn't as hampered by it (but still suffers some movement loss). That's my interpretation of it. If it's a weight thing, I guess he's just that much more buff than the others (but then shouldn't the barbarian have the least trouble?). Now its pretty clear that Borin's Armor is defined as being ultra-light (at least in the NA version) so the whole thing seems to be about weight as far as the game designers were concerned.
This reminds me of Legacy of Sorasil. I have never actually played the PC game, but from what I read in the manual, it sounds like every character can use everything, but the Wizard would not be as effective with weapons and armor and the Barbarian's magic would be really weak. Can't say I like that, I prefer the hard restrictions, to force players to make more compromises and work together, and because its nostalgia for the game I grew up playing, I'll admit it...
As Zargon I would think if it seems like the players are trying to cheat, so they can use long two handed weapons with a shield, and not be restricted by restrictive armor, I'd disallow it. Thinking about it now I'd say probably the Wizard should be hampered if he's carrying the plate to give to another Hero. Some players I know would just say "get it your dang self" rather than put up with the restriction. Subtracting only two movement seems like less of a penalty, but that second die always seems like it could be a 6, when in reality it could be a 1 or 2 anyway, from the player's perspective.
I have to laugh at the idea of a group of Heroes having one suit of plate and one crossbow between then and just passing them from one to the other every turn to avoid having to buy enough gear for everyone. But I've seen several types of player groups, mostly the dichotomy between the players who want to fend for themselves and let the other guy deal with the challenges and those who want to actually band together and do heroic deeds. The player who has to do a "costume change" every turn in order to get his right combination of gear from his "bag of holding" (bottomless backpack) is not far from it...