A quick note on Parry Hasting;
Whenever a Boss or Player successfully Parries an attack, their next Auto-Attack will gain UP TO 40% Haste. However, Bears are unable to ever parry, so bears will never gain this haste rating.
In addition, most bosses in ICC do NOT benefit from Parry-Haste. The only end-game bosses capable of Parry-Haste are Lady Deathwhisper, Sindragosa, and Halion.
Alot of people are under an illusion that Parry-Hasting causes extra Melee Attacks to appear out of nowhere. This is not true. For Example, if a Boss normally attacks every 2 seconds, upon successfully Parrying an attack, his next Melee will only take 1.2 seconds to strike, aka 40% faster.
A Few examples might be the best way to show how Parry Haste works;
Example 1
(0 seconds) Boss swings for 20k
(2 seconds) Boss swings for 20k
(2.01 seconds) Boss Parries an Attack, triggers Parry-Haste
(3.2 seconds) Boss swings for 20k (This attack took 1.2 seconds due to 40% haste)
Next Boss attacks would occur at 5.2 seconds, and 7.2 seconds, given no more parries occur
Example 2
(0 seconds) Boss swings for 20k
(2 seconds) Boss swings for 20k
(3.19 seconds) Boss Parries an Attack, triggers Parry-Haste
(3.2 seconds) Boss swings for 20k (This attack took 1.2 seconds due to 40% haste, but counts from the time of the last attack, not the time of parry)
Next Boss attacks would occur at 5.2 seconds, and 7.2 seconds, given no more parries occur
Example 3
(0 seconds) Boss swings for 20k
(2 seconds) Boss swings for 20k
(3.21 seconds) Boss Parries an Attack, triggers Parry-Haste
(3.21 seconds) Boss swings for 20k (More than 1.2 seconds have passed, so attack occurs instantly at the time of Parry. Note that LESS than 40% haste is gained in this example)
Next Boss attacks would occur at 5.21 seconds, and 7.21 seconds, given no more parries occur
Example 4
(0 seconds) Boss swings for 20k
(2 seconds) Boss swings for 20k
(2.1 seconds) Boss Parries an Attack, triggers Parry-Haste
(2.2 seconds) Boss Parries an Attack, triggers Parry-Haste AGAIN
(2.72 seconds) Boss swings for 20k (Parry haste stacks. 40% haste of 2 seconds = 1.2 seconds. Another 40% Haste to 1.2 seconds = 0.72 seconds)
Next Boss attacks would occur at 4.72 seconds, and 6.72 seconds, given no more parries occur
TL;DR, note that decreasing your chance to be parried against Lady Deathwhisper, Sindragosa, and Halion will slightly decrease your damage taken.