The less often you will be able to purify, yet your overall damage intake is higher. This may be fine in a 25 man environment, but not in a 10 man environment. What a lot of people do not realize because logs do not show it, is the amount of damage prevented from Purifying Brew. It doesn't show as healing, absorbs, dodge, nothing. I make it a rule specifically to only purify when the overall damage of stagger exceeds roughly 40% of my current health pool.
For instance: Let's say I have 500k health currently. I have a 10sec stagger where the overall damage is 200k. That is 20k per sec, not including the damage I will take that is still incoming. So the math would be:
100 / current health pool (500000) * total overall stagger damage (200000) = 40. I calculate the rough math using a tellmewhen and determine if I need to purify or not. I try and stick close to the 40% mark as much as possible. Obviously as my current health pool diminishes with subsequent hits, the overall stagger amount before purifying lowers.
Now to get back on point. With a high Mastery build, you are unable to purify as often, requiring your healers to heal that extra damage. Run comparebots with someone using an all mastery build and an all haste build and see the difference in shuffle uptime, purifying brew casts(have to manually do this in a log analyzer), damage taken, etc. I think you would be surprised.
Last night on Imperial Vizier Heroic 10 -
http://worldoflogs.com/reports/rt-nt7a1f36c60x6ytn/
34 Purifying Brew casts at roughly 40% of my current health pool while maintaining a 93% shuffle uptime. From a DPS standpoint, 3rd in the world. Could you manage that in an all Mastery build? Nope. Which would require harder choices from you whether or not to let Shuffle fall off to purify, or force the healers to heal through it.
It would be foolish to say that of those 34 casts was exactly 200k a pop prevented. But for simple math stake let's give a good range.
At best - 6.8m dmg prevented.
At worst - 3.4m dmg prevented.
Add those to your healing numbers and it is quite impressive indeed.