Haste increases throughput at the expense of increased mana consumption
Mastery doesn't proc on certain heals, the most noteworthy of which is EF, which is still the most popular talent for healing.
Int boosts base heals, which in turns boosts mastery shields. Boosts throughput at no expense of increased mana consumption, but not as random as crit since a significant part of its weight is tied to spellpower rather than its crit boost.
Also, we already get a decent amount of secondary stats off base armor this late into the expac. Enough for us to start boosting our base healing amount to actually benefit from these multipliers.