Like all true hybrids, Shamans are remarkably versatile. They can heal, they can melee, they can nuke. And like all true hybrids, they can only do one of these things, and then only if they properly gear and spec for it. While it is possible and worthwhile to really push the borders of the inherent multi-tasking of the class, most folks pick a role and stick with it, either because (as is my case) they don't have the gear to do anything more than their generally designated role or because they don't really want to. My resto shaman cannot DPS. Not even because of spec, but because he simply has not the gear for a DPS role and as a result there's no point in him attempting it. If I had good enough enhancement gear, I would probably try and DPS a lot more, but with the limitations of what he's carrying around in his bags it's simply not possible.
Your spec is like an aptitude: it defines what your character will have the most options in. As a resto shaman, I have plenty of added punch to my healing, talents like Healing Way, Improved Chain Heal, Earth Shield and Nature's Blessing. But I'm lacking the offensive punch of the talents from Elemental for my spells, which means even if I had the gear, my shocks and lightning bolts are not going to match up to a shaman who has spent the points. I won't have Lightning Overload or Elemental Precision. And if I try and mix it up in melee, I lack the ability to Dual Wield, the enhanced weapon buffs of Elemental Weapons or the bite of Weapon Mastery. I still have the basics that Elemental and Enhancement shamans use in combat, of course, and they still have the basics of shaman healing, but there is always a noticeable drop in performance when doing something outside of your spec.
Of course, some of you are probably saying "tell us something we don't know" while still others are ready to point out that even if an enhancement shaman can't heal as well as a resto shaman, he can still heal. And that's actually a good point. An enhancement shaman or an elemental shaman can, in fact, still heal. This leads to an often overlooked aspect of the class - basically, you bring a shaman along on a run because a shaman, in addition to whatever DPS or healing he or she can provide in a main role, can also provide what I like to call 'panic button' capacity.
It may not be accurate or fair to say that shamans are three classes in one, as the baseline abilities are shared between all three specs. All shaman can throw a lightning bolt, can interrupt a spell cast with earth shock, can toss a chain heal around or drop totems to buff the party or raid. The three specs make a shaman better at a main role, they don't turn him or her into a whole different class.
And that's a good thing.
No matter what role the shaman is intending to provide for your group, be he a main healer, a spell slinger or a in your face fist weapon swinger, the shaman is still a shaman. His totem buffs, despite their limitations, are among the best in the game and he can provide situational cleanse and fear removal, but beyond even that, the shaman can plug your party's holes for you. Do you need someone to interrupt heals on Romulo and Julianne? Maybe someone has to kite the main tank away from the party in Sethekk Halls because an MC went bad? Perhaps we need just a little bit more group healing on the DPS for the Maiden, or we're running Black Morass and someone has to do the adds and heal him or herself? Call a shaman.
No, a shaman cannot main heal, top the caster DPS charts and wade into the fray all with one set of gear, and even if she or he HAS three sets of gear there will still be discrepancies between a shaman specced for a role and one specced for another. But shaman still maintain a great deal of versatility no matter what they specialize in. They have a wide assortment of abilities and totems, even the awe-inspiring power of Sentry Totem. I honestly cannot conceive of how the Alliance killed a single boss past Drakkisath before they got shamans. I mean, how do you kill C'Thun without a Sentry Totem up? How does anyone even think about running Naxx without a Sentry Totem?
(The above paragraph on Sentry Totem was paid for by the Azerothian Society for the Promotion of the Use of Sentry Totems and does not reflect actual gameplay issues. Sentry Totem does not guarantee a Kael'Thas one-shot. Use of Sentry Totem is prohibited in several zones, check your local constabulary for details. It is ill-advised to use Sentry Totem to try and find out if Sylvanas is 'really that color all over' and Sentry Totem will accept no liability for any such misuse. Sentry Totem, unfortunately, does not contain a glowing substance that fell to earth in a meteor, and you'll probably feel free to taunt Sentry Totem. This will not, however, force Sentry Totem to turn and attack you. What would it even do, wave its pointless little arms at you? That's just silly.)
Okay, okay, I admit it, Sentry Totem is kind of meh. But one lackluster totem doesn't change the fact that the class really is much more versatile and flexible than people give it credit for. It's not all heals and DPS, although we're fairly good at that too. One issue I hear a lot from other shamans and read a lot on the shaman forums is that people often don't want to bring shamans to five mans, especially heroics, as anything but healers because of their lack of CC. And it's true that we don't really have anything more than earthbind and frost shock, which are more suited to kiting than to actual crowd control. The dev's have stated that if we do get any CC, it won't be before WoTLK, so the fact is, we don't have it and we won't have it. But hunters, rogues, warlocks, druids and mages do, so here's an idea: bring two of them and a shaman, and you're good. Does shaman versatility entirely compensate for this one lack?
I don't know, although I would obviously have to admit to being biased enough to think that, yes, it does. The ability to make whatever other DPS in your group exists better, the ability to offheal if a pull goes bad or if a boss needs a little more healing than one person can supply, the ability to adapt and drop situational buffs to specific conditions in a fight... the shaman's real strength is what it provides that isn't just DPSing or Main Healing, all the other aspects of the class. No matter what his spec happens to be, having a shaman along makes everyone else in a group that much stronger, and when he's geared and specced for a role, they can do amazing things. Heck, just for Bloodlust/Heroism it's often worth it to have a shaman along, not to mention their Fire and Earth Elemental Totems.
Maybe not three classes in one, but certainly one class that can do more than three things.