Let's say hypothetically that a healing class was severely lacking in mana regen. So much so that they were going OoM very early in the fight and therefore could not get healing to the target. Obviously, this situation would require an immediate fix to improve their regen and give them the ability to get their healing to the target. Shaman regen is fine. But we are facing a similar problem. We do not have the ability to transform our mana bar into meaningful healing for our raid group. This is due to absorbs and other instant and aoe instants sniping the healing headroom. This makes our mana bar less useful to the raid's survivability compared to that of other healers. That's why you're seeing Shaman stacking Spirit.
I also wanted to requote something Tiberria quoted in a seperate thread.
-Explanation from GC as to why Resto Druids were buffed in 5.3/5.4HoTs used to be the mechanic that provided some healing “cushion” and now it really feels like absorbs have shouldered hots away. We buffed Resto druids a little in 5.3 and we have plans in 5.4 to emphasize their hots even more, especially in AE healing situations, which have become the dominant way to heal raids. We’ll see what the 5.3 changes bring to absorbs and make further changes as necessary.
GC is of the opinion that if a specs heals are getting sniped, that is a problem.