Actually, a full set of plate armor usually went without any chain underneath. I agree, plate was expensive and time-consuming to build, but this should probably be covered in another thread. Most nobles who could go to war would go with a set of armor fitted to their liking. Otherwise, the armorers would make peicemeal plate and chain (which was called transitional armor) for the grunts, or the grunts would go unarmored.
Plate armor is actually not incredibly heavy--only slightly more than the gear a soldier lugs around IIRC. Demonstrations with modern people in plate armor jumping, falling down and getting back up, jogging, and more have been made--it is reasonable to assume that a warrior "caste" would be very comfortable in plate armor.
I could argue about the finer details of this, but that's not really the point of this topic--this topic should be about weapon damage.
Another few points to consider are how the wound might heal--for example, stab wounds were notorious for becoming nastily infected, if the wounded survived the initial stab. This was mostly because the wound couldn't be flushed out with antiseptic or whatever the term was at the time. Also, the term "infected" was used quite loosely, so it could have been also a complication rising from mixing fluids that weren't allowed to drain properly.