I suspect it depends. I get the impression that certain abilities are OP, they'll nerf them specifically. In the case of warlocks there was a couple of buffs and nerfs that fell outside MINUS TWELVE. You can't hit accross the board if you want to buff certain spells or hit certain spells a lot harder.
When things are really out of line rather than just a bit out of place, a more thorough, structured and targetted change is the only real solution, anything else is just stalling a bit.
If a class is just accross the board OP or weak they can hit the talent bonus. They won't hit the mastery unless they want to change the character's coefficient to the mastery stat but instead hit the non mastery bonus. I suspect they actually meant the passives built into the class and not the mastery itself. While mastery affects scaling, if it's not the best secondary stat, nerfing it won't hit scaling hard. Like "cataclysm" for destro warlocks. Which is not a mastery but a passive. The passive therefore is a good tool for wide reaching nerfs at all gear levels. Whereas changing coefficients, and talents will affect scaling more deeply. If they just want to alter classes at a certain point the passive nerf/buff is a perfect band aid but not a good solution.
In the end it's just another tool, which means they can react quickly and fix things, but one they will not always jump on ESPECIALLY around major patches. If they relied on passives, classes who scale well, like warlocks, would have to be nerfed every patch forever, when nerfing a coefficient or talent might work.
I feel bad for locks and mages, their mastery is just a copy of the passive, and compared to other classes they don't get many passives or active skills from picking a tree.