Okay, so as long as I can remember playing a druid (which is a long time), innervate has been on a 6 minute cooldown, and it restored a big chunk of mana. So, if you knew it was going to be longer than a 6 minute fight, you had to decide whether to try and innervate early, in hopes of it being up again, or you just innervated once at the mid-way point, or whenever it was that you got dangerously low on mana from healing. If it was going to be less than a 6 minute fight, you just used it whenever you (or someone around you) was low on mana, whenever that happened to be, without any regard for having to use it again. And, since it restored 3/4ths of your mana pool,
Now, on a 3 minute cooldown (in 3.2), restoring half the amount of mana, we need to use it earlier, so that it will be back up before the fight ends. It also means that since it restores half the amount of mana, we’ll also need to use it more often. So, you’d have to hit it early in the fight, and then hit it again once it’s off cooldown 3 minutes later. If it’s a long fight, you may even be able to squeeze in a 3rd. In a 5 minute fight, you could use it at min. 1 & 4, and not really gain any more mana than you had before. But, it also means you have to time your innervate, since using it at minute 3 of a 5 minute fight means you only get to use it once. So, it’s really not that much of a PvE buff, given the normal length of boss fights. In a 10 minute fight, you would get to use it 3 times if you timed all the innervates right. Keep in mind that even as a resto druid, you don’t have to use it on yourself – now that even paladin healers are complaining about possible mana problems, and replenishment got nerfed, our mini-innervates are going to go a long way (maybe).
This means that feral druids can cast it on people multiple times in the fight if casters start having mana problems. Moonkin can use it on themselves or on other people, and in most fights, they can use it on more than 1 person if they want. So, in conclusion, we’re going to need to time our innervate useage, so that we can maximize the mana gains. I’m pretty sure other classes are having to time their abilities that are on shorter coldowns, so I don’t think it will be that hard of a switch.