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.
Merlot concludes that the only real option is to throw the whole thing out -- along with the Replenishment changes in 3.2, the MP5 change means they're already heading back to tweaking individual classes (classes like Shaman and Paladins focus more on MP5, and thus are targeted there), so they might as well just do that for everyone. Otherwise, they'll soon realize that the current Replenishment numbers offer too little mana to some classes (Priests?), which will lead to another Replenishment buff and a nerf to something else. Blizzard was trying to save some balancing by making everyone responsible for Replenishment, but I'm with Merlot: it's a clumsy way to do things, and it leads directly to this back-and-forth balancing, where you're targeting the problems of the solutions rather than the problems themselves.