It does seem like the least Kirian could do is install the addon and see for himself what type of advantage it provides before labeling 20% of the community cheaters with such strong language. I'm saying 20% because the addon has 1500 users and there's 8000 on the leaderboard. Wouldn't shock me if that 1500 skewed towards the higher end of rankings and towards those players who played more games, so the game-weighted average might even be higher than 20%.
The add-on just doesn't matter in the majority of everyday (2 player) dominion. I sometimes sign in on firefox by mistake, and might not notice for a full game that I don't have the addon available. If you play a lot of dominion, you are going to remember all your important cards, and as tim said earlier in the thread, by subtraction, you know what your opponent holds.
I suppose how much it really matters varies from person to person, but for me personally, it really only helps with vineyards, silk roads, sabateur, or swindler on the board.
The recent 4 person qualifier is a different story, however. I found it extremely helpful, because I cannot remember what 4 different people are buying, particularly when people are playing slowly and the game drags out and approaches 30 minutes in length. So I can understand the opposition to using it in that format.
And to agree with what blueblimp just said, its baffling to see such surprised indignation at the use of the addon, considering that if you play a lot, you see the link to the addon spammed in your chat window several times a day. The reaction we're seeing suggests that all this cheating was going on behind everyone's back, when in fact we link the addon at the start of every match and its only a 1-click install.