How will this increase good gameplay instead of boosting agi ninjas never having to fear the bounces anymore? The only way a plated knight with a slow weapon can currently kill an agi ninja is by hoping for a bounce. Blocking will not help, because the ninja has 160+ WPF, while the knight has maybe 110 with weight penalty, and a slow weapon, not a 100+ speed katana or warspear.
Actually, I think if armor was buffed, it would lead to more teamplay, because the tincans would have to be approached by players with anti-armor weapons, while those with weaker weapons would have to guard them against other opponents. It was a bit like this before the upkeep system, plated players were only approached by those with blunt or pierce damage (or other plated players). The problem back then was that everyone had plate. But this has been dealt with by the upkeep system.
And BTW, you said: "You just can't spam all day and hope everything glance off your plate." This sentence either implies you can currently spam all day and hope everything glances off your plate with any success (which would be wrong, because only very few hits currently bounce off any armor) or it is stating an irrelevant fact. Also, spamming as tincan will kill you very fast, because you don't have the speed to keep enemies at a distance.