When Mastery was first introduced it was going to be the "tuning knob" for classes. The stat that Blizzard could adjust up and down easily to quickly tweak damage, healing and defensive numbers to quickly balance classes and specs, at one point in time Mastery was a really cool stat for a lot of classes too.
The stat failed at the intended goal of being that tuning knob though and instead Blizzard introduced the incredibly boring stat of Versatility to the game and made pretty much all the masteries "do X more damage of Y type or during Z." Which wouldn't be so bad except that the damage type was either so rare that it didn't really matter to the rotation or the amount of mastery required per point of damage increase was stupidly huge to the point of being irrelevant compared to other easier to obtain stats.
Meanwhile they added in straight class auras that handle damage and healing that all figure versatility into them and while the general rewards for stacking vers for most classes are like getting socks for Christmas there's no denying that practicality can win marathons.
So is there really any point in retaining Mastery at this juncture? I can't really think of any class that favors it over any of the other secondary stats and how most of the heavy lifting it used to do has been picked up by Versatility.