This idea came to me while I was doing a daily CM on my ret. Let's say you're a raid leader and you're wiping on this boss. This boss is hard. You're standing out of bad stuff and using CD's correctly, but your healers are undergeared or something and it all goes pearshaped. You need some off healing. So you replace one of the DPS with a different class to try and give you that extra edge. This scenario might get more common with the changes to healing in WoD.
So let's say the reason you wipe is because the tanks die. The tanks keep dying to this one ability that stacks up or something, and they can't survive both the deadly ability and the boss's attacks, they just die too quickly. If you were to have a DPS guy help the tanks on this, you wouldn't ever really take a shaman for the off healing. You'd take a ret. The ret can still do his instant 100k+ Flash Of Lights with selfless healer, gets even bigger crits if there are any adds he manages to snipe, has two massive damage reduction abilities with a tiny CD to use on the tank, and a Lay on Hands to instantly get that tank back up to full hp. The ret would be the clear winner.
If there was an ability that bursted the entire raid down with massive, unavoidable AoE (Think Galakras), you wouldn't take a Shaman, you'd take a druid and have them use Natures Vigil (Which is like Ancestral Guidance, but with much better % of damage converted into healing on a class that does more damage to begin with).
And if there was a slow whittling down of everyone due to raidwide damage, Ysera's gift would do that for you.
I really don't know here, in what situation would the offhealing of a Shaman be preferable to the offhealing of the other hybrids? Am I missing something?