I don't believe that plate is an overwhelming advantage that cannot be overcome but there's a reason why before the big january patch, most experienced players rolled plate. Considering the fact that average damage has if anything gone down, due to the lack of many players higher than level 31, and the fact that armor heirlooms have been buffed, I fail to see how the situation has changed other than upkeep. Certainly, one can make an argument that upkeep balances it, but I find it hard to believe any argument that heavy and particularly heirloomed armor is balanced compared to light armor without upkeep. The advantages in mobility are useful and more importantly to me, fun, but they are mostly negated by certain factors. Water. The difference in speed traveling through water is so miniscule that there's no logical reason to have ath if every fight took place in water. Hills. Unless you're traveling at certain angles, slopes can once again reduce the difference in speed between high ath players and low ath players. Rain. It does the same, albeit to a much lesser extent.
So, we have situations where having movement speed as your advantage can be partially negated. Where is the advantage of heavy armor negated? I cannot think of a situation where taking less hits to be killed is a disadvantage. The disadvantages of armor, having less speed, certainly makes a rather significant difference. However, my personal opinion is that it makes less of a difference than taking more hits to kill provides. No matter the skill, it is relatively easy to take the occasional hit. Teamhits can be awkward to block, if there are enough ranged it can be awkward to dodge, and if you don't notice a ranged character well, you won't be dodging. The difference between plate and agi is that those occasional hits are much more threatening to the agi build. When I wear light armor, teamhits make me rage a lot more than when I wear heavy armor, due to the fact that they take off a very significant amount of health.
Oh, one thing to mention. Can people stop bringing up the fact that they topped the scoreboards as a certain build or using certain equipment? If you play enough and switch gears enough, you're bound to top the scoreboards with just about every build. Hell, topping the scoreboards isn't even hard. Score means NOTHING because it doesn't take into account who you fought and beat, and whether or not you actually contributed anything to the team. If you are killing the same six level 1-5 characters every round you'll end up with 24 kills at a minimum. I don't really feel like killing 6 peasants contributes much, but hey, the scoreboard certainly thinks it does.
On another note, I'd like to mention that although my current setup is rather light, I've also gone through quite a few gens where I wore quite heavy armor. Including a gen where I wore triple heirloomed Rus Scale with Lordly Hourglass Gauntlets. I don't wear heavy armor like that nowadays mostly because it bored me. I don't enjoy moving slowly as much as moving quickly and I don't really enjoy killing people because they glanced on me, or getting hit 3 times and then winning because they couldn't take the same 3 hits. It just doesn't do it for me.