In one of my first posts I talked a bit about "healers doing damage" and how I could see some interesting uses for Telluric Currents, and that was back in the beginning of Cataclysm, but since I changed to a holy paladin just around the time my guild started to progress on hard modes I didn't really get to examine the usages of this spell. However, I am now back playing restoration shaman full-time now and while my guild is slowly progressing through Firelands, I want to give my point of view on Telluric Currents for what encounters it will be useful, and not downright dumb to take the talents.
This is from my experience with the Hard Mode encounters.
Shannox: Hardly any use and because I am healing the person who is getting raped by Rageface I have very little "free time".
Lord Rhyolith: At first glance it would be a stupid idea to spec into TC for this encounter because of the damage reduction, and when the damage reduction is reduced you end in a DPS / Healing race. However, you can get quite a lot of the talent if you can communicate with your fellow healers, and here is why: The Spark of Rhyolith (I cannot recall if this is the same on normal, however on HM this does apply) stacks a buff on themselves called "Infernal Rage" and it increases their damage dealt and taken by 10%, and if you can allow yourself to throw in a few Lightning Bolts at +6 stacks you will get quite a lot of mana back. However, I would only suggest this if you got very good communication between the healers.
Beth'tilac: I am usually upstairs so it is not because I have a lot of time not casting GHW on the tank or CH on the melee crew we got up there, I might add that Riptide is golden there and it just gets better when you get 2-set bonus. However, when we approach phase 2 and she gets down on the ground, perhaps I should add that we run with either 2 or 3 shamans on this fight and it is mainly due to players at our disposal, and we pretty much just keep Riptide in the tank and Healing Rain on CD and then we push as much damage as we can with Fire Elemental and Lightning Bolts / Flame Shock / Lava Burst in order to make the fight as short as possible and thereby less likely for us to wipe. Our first kill was with the three shamans doing between 700.000 to 950.000 damage each which might not seem like a lot, but that is 1500-2000 DPs that we each were able to dish out, and whenever you are progressing on a boss you want to keep your DPs as high as possible, and a shaman in capable of doing so with very little effort and will gain something from it (If you spec into Telluric Currents).
Alysrazor: Telluric Currents isn't really mandatory because of the "regeneration phase" in which you will be topped off after a few casts, and mana is not really an issue unless your melee plays "HOW MANY BRUSHFIRES CAN I HIT" or "INTERUPT - WUT IS DAT ?". Nonetheless, because of the many targets Hatchling, Meteor and Druids I will strongly recommend to at least try to keep Flame Shock up in the Hatchling and possible Alysrazor whenever she flies by if you got mana for it.
Baleroc: We are currently progressing and after a 12% wipe the first night I do think that we should be able to kill him with in a raid night, however, it is brutal. Almost constantly spamming GHW to keep people with Tormented up and the tank taking just as much damage (I love whenever our tank dodges / parries the majority of Baleroc's Decimating Blade). This might be after his nerf, however, I will say this - Two restoration shamans is NOT optimal for this encounter, and I cannot say that I have found a use for Telluric Currents here simply because the damage input is so damn high that you cannot just say "All righty guys - I am going to regn." at least not without taking a rather big risk, and penalizing your fellow healers because you had to be a little to aggressive with Healing Surge to get a few more stacks.
Majordomo Staghelm: I have only had a few wipes on this boss, and haven't really been pass the 70% marker, but with the damage and healing debuff TC might come out quite nifty, but I will make a repost when I know what I am talking about.
Ragnaros: 2-3 Months and you will have an answer ;)
However, to summarize - I do think it is immensely important for healers to squeeze out a little damage if they got mana, and furthermore if it does not jeopardize the tank / themselves or a raid member (DIEEE DKS DIEEEEE), and if you expect a damage dealer to use defensive cool downs / spec into damage reduction talents and optimize spec / gear and gems in order to squeeze out a few more DPs - Then should they not expect you to throw up a Flame Shock, Moonfire, Insect Swarm, Devouring Plague to gain 100 DPs or more ? (Yeah, I know I "forgot" paladins, but they get raped whenever they attempt to do damage with Exorcism)