If You Were A Class/Job Designer...

[Violette]

However buffed version of dark dance is what makes DRK actually quite lower in terms of tanking than WAR or PLD.
20% evasion? Can anyone confirm, that lvl 60 dungeons or some kind sephirot trial has evade chance on at least 1%? In dungeons you may evade 2 or 3 hits from big pulls, but i don't know, if it's worth exchaning for DA + Abyssal Drain, since you will just self-heal yourself for sure, not a probability chance.
What's more, buffing evade by 20% doesnt mean we have 20% chance to evade attacks. We buff evansion stat by 20%.
And since DD increases parry by 30% not 50%, math is telling us to use something stable, that won't make healer's cry "use cooldown!".

"B-but i did... Dark Dance..."

Actually 20% increase in evasion in this game IS a 20% increase to evasion (flat), not multiplicative (as far as testing has shown). (Same with parry/block increases. Testing this at the moment in case I've momentary insanity).

Just as 30% parry increase is 30%+base+other sources.

Also, theoretically a 30% increase in parry is a 6% increase in physical defence, which can swing high/low. As we approach n=infinity, where n is number of attacks incoming, this will come closer to the true average.
Not to mention 20% evasion is (again theoretically) a 20% increase in defence against all avoidable attacks. This puts in on par with shadowskin (on a lesser cooldown), with the added parry bonus which adds to low blows/reprisal.

Now while we must recognize the lack of ability to normalize these % chances so they always happen X no of times out of Y tests, we can still see that these abilities approach their true affectiveness under 1 of 2 situations:

A) Multiple mobs are dealing physical damage to the dark knight

B) Mobs (needs not be they have many), are using rapid attack moves that hit quickly. These could be things like Bahamuts claw, Uplander Doom, etc.

In both cases, DA-DD will approach it's true effectiveness.

So while people may rag on DD all they want, it's still the most effective tool Dark Knight has for multiple mob situations.
Am I advocating the use of this for busters such as double rocket fist?
Of course not, but as you've so eloquently put it....."math is telling us", DA-DD can be more than useful as a replacement for shadowskin against multiple mobs.