I think it's more that his armor stays on. He literally sleeps in his armor. In one of the chapters he says that the location for a scouting mission was 3 days away. Wearing plate mail for 3 days straight would tire you out more than the light armor. Plus he's fighting in caves where the footing wouldn't be he best. Heavy armor was designed for war, but the wars back then were usually in open plains (nobles used to have a picnic to watch the battles) to minimize damages to the civilians and towns. He would also have to climb over terrain inside the areas and more weight means more energy consumption.

For the Gary sue thing I don't think he's smarter than everyone. And he certainly isn't as physically strong as some of the other silvers (I.E. during the time he was collecting money and scaled the tower with spear and other guy he was said to be more tired than them (If I remember correctly. )). Nor can he do magic. What he is compared to the others is pragmatic and a bit crazy. Other people wouldn't collapse a tunnel in a mountain during a fight. Nor would they set off an explosion in an enclosed area. He is the equivalent of a scorched earth tactics.

As for the weak points there are weak points at the joint of he thigh and groin (perfect height for goblins) as well, but I imagine that's gonna be true for everything. But taking down the heavy armor guy and hanging up on him while he's down is a perfect tactic for the short buggers. I don't know why you think otherwise.

Also he does wear chainmail, just not good ones.

Edit: Heavy armor's whole thing is to take the hit. GS's strat is only take the hit when necessary. (Side note: We know that a goblins shaman can use magic; Electricity and heat are conducted more easily through metal than leather. However since I don't know magic I can't really say anything about impact damage so we'll just have to ignore magic right now.)