Ah, Replenishment. No buff might be more welcomed in raids and reviled in theorycrafting. Ever since Ghostcrawler told us it was a necessary buff earlier this year, Blizzard seems to have twirled it around and around, taunting us like the proverbial carrot on a stick. It's been passed out to many classes, buffed a few times, nerfed even more (that Arena nerf was particularly strange), and in patch 3.2, soon headed to the PTR, it's getting nerfed again, even while MP5 (mana per five seconds) is getting a boost. What's the deal?
Merlot, the Shadow Priest behind the Misery blog, has a good breakdown of just why Blizzard is so schizo with Replenishment. The whole point of the buff was to have Blizzard have some control over mana during fights -- instead of worrying about each class' mana separately, they'd just have this buff that gave mana like a big spigot, which they could then control as they saw fit. But players are so different across the board that putting them all under one big buff umbrella hasn't worked so well: a buff to Replenishment helps some and hurts others, and a nerf does the same, meaning Blizzard is flipping back and forth on turning the spigot on or off nearly every patch.