Don't be afraid to try, though: the attempt will often endear you to groups, and when you actually manage to succeed at it, people will remember you. I've had people comment on my 'recovery mode' skills on more than one occasion, as it's something I happen to have developed a fairly good eye for, partially from having played primarily as resto for so long on my orc. It doesn't really demand too much from you as a DPS shaman, just an ability to occasionally look at health bars and make use of what you have on the table.
First off, learn to know when healing stream is more useful to you than mana spring. I know it doesn't seem like much healing, especially when you're not wearing the +healing gear to really amplify it, but sometimes healing stream ticking away can keep you from having to worry much, if at all, about the rogue or hunter or mage or warlock or other DPS in your party. If you can focus your efforts on less people, you can make the most of your limited healing viability. However, do yourself a favor and always keep an eye on the main healer.
Main healing can be a stressful job and often, healers forget to look after their own health bars. While you shouldn't be flinging heals at the MH the second he or she drops a few points, it is never a bad idea to throw a chain heal or healing wave into the MH rather than letting him or her drop trying to heal while being interrupted by damage. This requires you to stop DPSing, yes, and I know that the other DPSers in the raid will not understand why your damage suddenly plummeted and may well give you grief for it, but you'll just have to ignore the kinds of players who are owned by the meters. Stopping DPS to keep the healer alive long enough for the tank to regain aggro not only prevents wipes, it prevents you having to go into full-on healer mode in your damage gear.
Another situation that can arise, specifically for an enhancement shaman, is when you cannot go into melee range for whatever reason. Perhaps the boss has a poison cloud or bleed effect, perhaps you're getting unlucky on Flame Wreath, perhaps you're rooted or in ice chains. (Okay, so I'm having a lot of bad luck on Aran fights.) For elemental or resto this is of course not so big a problem, but for you, it's still better to throw a shock or a lightning bolt than to just stand there doing nothing, and it never hurts to throw yourself or even someone else nearby a heal. That rogue about to die from arcane missiles won't mind and the healers aren't going to complain if you throw a couple of chain heals around while you're waiting to get back to punching Medivh's dad in the back of the head.
Keeping in mind that we're talking about panic button uses of shaman abilities so far, let's talk about how best to use specific abilities for these situations. Although it's usually best to conserve this for a boss, if things go south on a run there's no shame in popping Bloodlust/Heroism to help jump the group's DPS and help you cast those emergency heals a little faster (the GCD will remain constant, but at least your big heals will cast in a shorter amount of time, useful for when you're not a resto shaman with imp healing wave and have the full cast time on them) - it's also not a bad idea to drop one of your elemental summoning totems in this situation, as preventing a wipe can often help keep your momentum and is sometimes worth not having a pet for a boss fight. If for some reason the tank drops, earth elemental totem is a useful backup. We found that out in heroic Underbog two nights ago when our tank, and half our server (Norgannon) disconnected while we were fighting a three wasp pull. Suddenly, my elemental and I were all the tanking that group had, as I had to switch to frost shocking one bug off of our priest while the EE tanked the other two for us.
Also, it really doesn't hurt to try and learn how to gauge when best to use reincarnation, and how to use it. Don't just think of it as wipe recovery, as I did for the longest time. If you go down, it may be best to stay down, but if you can see that things will be manageable with just a little help from you, go ahead and pop up, throw whatever totems you can afford (improved reincarnation can be a big help here, if you have it) to give the group the buffs, and look for where you can best serve, perhaps going right back to as much of a DPS role as you have mana/health for, perhaps throwing a few backup heals, perhaps bandaging your low-mana butt first. This is one of the tricky aspects of the ability to self res, and I know a couple of shamans who have honed it to an almost psychic ability, knowing exactly when to pop to keep fighting vs. when to stay down and help a group recover.
We've talked primarily about emergency fill-in roles and how to use your abilities to deal with panic situations. Now, let's talk about something far less urgent but which does happen from time to time: how to play a resto shaman in a run when you're not the healer. Much like bringing two tanks to a run, two healers is an unusual situation and not many PuG's will really want to do it. Generally, a PuG will just not take you if they have a healer and they find out you are resto. However, for good and for ill, guild groups and groups of friends often are much more willing to take... or even drag... you to an instance as a fifth man even if they already have a tank, two DPS/CC and a healer. In this case, just go along with it. How often do you get to relax and play up your hybrid nature?
My advice here is to break out whatever DPS offset you've collected that plays to your strengths and is the most effective. Generally most resto shamans I know tend towards preferring an elemental playstyle to an enhancement one (I'm different in that I love enhancement and will always choose melee over casting if I have a choice) but with recent talent changes, you can always strap on melee stat mail and throw on a big 2h weapon. If you're gear is solid enough for whatever role you've chosen, you can find yourself actually performing solid DPS and your totems, while not talent improved, are still some of the most solid group buffs in the game. You're not going to out-damage a dedicated DPS shaman, but you can contribute, and no one ever says they hate having a backup healer who can actually heal if it comes down to it. My orc has actually managed to come second on the DPS with his big 2h axe when windfury decides to proc a few times. It's not a role I'd actively choose for him, but it's a nice vacation from healing from time to time. Don't decide you can't possibly do it just because you can't do it as well, sometimes folks just want to go to the instance and aren't as hung up on everyone being perfect.
Basically, this column, like the one previous, is me really focusing on the specific way shamans are a hybrid class and how to best make use of it. It never pays to forget all the tools you bring to an instance run, even if other players often seem to.
Next week, who knows? I am as always interested in hearing what you'd like me to focus on. Elemental DPS strategies? How to pick a spec? The best shaman talents? An in-depth discussion of why there are so few 2.6 or slower offhand fist weapons and that's why my shaman is wearing what looks like C'thulhu's oven mitt in the picture? The future is as always as much yours to decide as mine, gentle readers.