While I understand what's going on re: Disc Mastery, the same could be said of other masteries (e.g., resto druid, holy priest).
Holy priest mastery, for example, affects all of our spells that proc it in exactly the same way. How excitiing. Haste, on the other hand, affects different spells differently. It affects the cast time of cast-time spells, the global cooldown of our instant casts, and how often Renew ticks (but not Lightwell Renew, HW:Sanc or GoPoH). Crit affects all of our spells to the same degree, but Inspiration causes some of these spells to have an additional effect, so even crit is still above mastery in terms of how "interesting" it is as a stat.
A resto druid has a flat increase. Also very boring.
A holy paladin adds a bubble, but is still limited (within reason), and still doesn't proc off some heals, which is unfortunate.
For a disc priest, however, it affects one actual spell, and one proc, which relies on either another spell (PoH) or another secondary stat (crit) to make it useful. To this end, it needs to affect some actual spells. I'd most likely vouch for Penance and Atonement to be included into Mastery in some way (either increased healing done, adding another bubble, or something), and perhaps in some way Barrier (though how to manage this without making it so far above par versus a holy priest than it already is, or completely unbalancing PvP, I do not know).
Now, since crit affects mastery procs, this makes it attractive in some circumstances, which is good. But the problem is that, in and of itself, it (mastery) does not scale with any stat other than intellect. In their own ways, each healers' masteries scale from intellect to a degree, and will always scale with crit in some manner. However, each healers' masteries also scales from more than one spell and a proc, except for discipline's.
Unless I misread this when blizzard posted it multiple times, they didn't say that at all. They said they don't want disc priests thinking they should ONLY be casting shields. They want the focus to be on shielding but still require them to use spells outside of just PW:S.
I'm fairly sure "spells outside of PW:S" equates to "only sometimes [using PW:S]" in fancier words.