Shakes

Shaman are weak single target healers, but not weak tank healers, if that makes sense.

Where as other classes heals are largely wasted as overheal when spam healing the main tank, chain heal (while healing for less) also tops surrounding people up, so as others have mentioned it's actually a very effective tank heal in its own way.

Earth Shield is also a very efficient tank healing spell: sure it's not huge HPS nor stackable, but it very rarely overheals, so you know you're getting something like 5-6k healing (depending on gear) for your GCD and 450ish mana every time. Doesn't show up against the shaman on a WWS, but if you drill down, earth shield is often doing around 10% of my healing, or around 5% of total tank damage taken if you want to look at it like that. Not bad for a fire and forget, just refresh when you have the time spell.

And in any situation where you have two or more tanks standing close enough together for chain heal to jump between them, shaman suddenly become very high throughput tank healers, despite not being good single target healers.

Druids as raid healers do have issues. I think the basic idea isn't completely wrong though. Their strength is that for the many fights with only one or two tanks, they can also heal the raid a bit while maintaining their effectiveness at healing the tank. More than anything, I think a druid is a healer who is most effective when they have a priority list (keep all hots up on there targets, when you have the GCDs in between that, help out on these other targets). Although they're "tank healers", they're not like paladins just spamming their heal over and over on the tank.

And as much as has been said about HoTs not being good for raid healing, I think the basic idea for a druid is to not always target the lowest health people (direct healers will do that, not to mention chain heal jumping to them), but top up those who might need a couple of HoT ticks.