Avoidance really isn't as bad as most people think.
Calling it Russian Roulette is unfair. Luck plays somewhat of a part in determining how effectively the tank tanks, but is a lot less than people think.
Imagine this hypothetical scenario.
Say a guy has 50% chance to avoid damage. How often will his health spike, and how often will he conserve health? It would be about 50-50. So every time he gets hits, it's like flipping a coin. Heads, he gets whomped. Tails, he takes immune.
On the other hand, what if the guy had 100% chance to avoid damage? Well, in THAT case his health isn't going to be spiky at all. In fact, it's going to be just the opposite. It's going to be completely static. He'll resist all damage, all the time, and the coin flipping goes away completely.
By mathematical standards, that's called entropy, or the study of static variables as the result of probability. Probability isn't always a coin flip, and getting 60-40 on 100 coin flips is unrealistic, because that requires you to get heads 50% more often than you get tails.
In ICC10 gear, Dodge + Parry for a DK, both of which I'm guessing are at about 65-70% total, plus raid buffs, we're talking about a very static intake of damage. To be hit by an attack more than twice in a row in a full 6 minutes of fighting is probably extremely unrealistic, and will most likely never happen. If it does, that's what PW:S is for
I'm not saying health isn't better overall for tanking. It's just that parry/dodge are valuable enough on their own.