I find with Zuato that I tend to innervate myself during the long boss fights vs. a priest because as it stands now most of the priests we go to the core with have higher Spirit and more +mana/5 gear than Zuato does, so they don't run out as quickly. Now that I am getting more +Int, +Spirit and mana/5 I do innervate priests more often. The other reason is that the priests have been doing more de-magicing leaving us Druids and Shaman to do the healing on the MT and parties.
As for innervating a resto spec Shaman that is assigned to a tank for a boss encounter, yes, I would innervate them if necessary since they are a primary healer. It's amazing how Blizzard made only one class Horde side with the ability to de-magic, yet give their tertiary healing class limited healing compared to the Paladin...so we have to put priests on de-magicing duty and have the Druids and Shaman heal. Not that big of an issue really, but shaman have the WORST mana regen from spirit of any caster/healer class in the game. I can sympathize since I have both a 60 Shaman and Druid.
That said...we've suffered a shortage of Druids with the exception of one night in our MC runs. For a long time only Ionas and I (regulars) had innervate, as we were the only two druids that attended regularly that were fully restoration specced. Now that it is trainable I can see us using it more, however we usually average 2 to 3 Druids per run as is, so we have to be picky about who gets innervated at this point in time. We've even had a shortage of Shaman lately too, but fortunately we received an influx of priests to help compensate.
I do love the swiftmend talent. I can pop a rejuv on, swiftmend and immediately throw another up for a lot of healing fast.