Also, assuming you can reach 60% avoidance, 6% chance to get hit 3 times in a row means that on a boss fight it is very likely to happen. Let's just use an imaginary boss that hits for an average speed of 2.00 after calculating in parry haste and thunderclap. Let's say he takes about 3 minutes to DPS down. That's a total of 90 swings, and we'll separate that into 3 swing segments. That means 30 3-swing segments, and a 6% chance for one of those to be a 3-HIT segment. That means at least twice, almost guaranteed by the percentages, you will get hit with a 3-hit streak.
Now, if you have geared for avoidance so that you manage to somehow reach 60%, you're very likely going to have very low EH which means that this inevitable 3-hit streak will kill you, or bring you dangerously low.
Moral of the story is that a healthy balance is necessary. It's tough to focus purely on EH because your hitpoints (as a warrior) will never be high enough to rely on exclusively, and it's tough to focus purely on avoidance because that will never be enough to rely on either.
Going back to the original post, what he said is all true about the 1% at 50% is more like 2%, but it doesn't really mean much because of the fact that you really aren't going to reach much higher than say 55%.
I am speaking from a prot warrior standpoint of course. I won't get into how druids can pull off way more EH/avoidance...