In 4E, leaders do a whole lot more than just heal, though all of them retain a basic ability to heal thanks to their 2/enc. class feature. Obviously characters with more focus on healing are free to take other powers that heal as they level up. Conventional wisdom suggests that other (encounter) healing powers should be used first during combat, saving the minor action 2/enc. powers for emergencies (and any Daily healing for even more dire circumstances). This is because they're burst 5 (good range but won't provoke OAs), minor actions (so you can do something else with your standard action), and usually provide a substantial burst of healing (they general formula is surge + xD6, where x depends on level). All good things for when you're in a tight spot (other forms of healing may suffer from being ranged (will provoke OAs) or even melee (touch), requiring that an attack hits, etc).
For more general guidance, check out this thread from the WotC forums. Here are a couple of points that the guide makes (on the subject of when to heal), which all support the notion that class feature 2/enc heals should be saved for later:
Rule 1: It is better to heal later than earlier.As long as your ally's are up in the safe zone, it is better to keep healing powers available in case of an unforeseen situation, such as sudden monster reinforcements, or a second encounter before you can take a short rest.
Rule 2: Surge-free healing before surge-cost healing.Healing surges are your main daily resource, and saving healing surges is the main focus of 4th edition resource management (after staying alive). If you have an encounter power that provides some free healing on the side, use it first.
Rule 3: Healing Strike before Healing Word (and similar)Attack powers that heal help decimate the number of monsters, so use them early. On the other hand, they can miss, so they are not reliable if an ally is down on the floor. That's why you should reserve your minor action class feature heal for emergencies.
Rule 4: Encounter before daily.Encounter powers that provide healing will be back after a short rest, so use them first. Daily healing like Cure Light Wounds should be reserved for encounters where you need more than the normal amount of healing.
Rule 5: Temp hp first, healing later.Temp hp go away at the end of the encounter, so use everything that gives temp hp while there are still monsters around.
Now that it's (hopefully) been made clear that class feature heals should be saved for when they're really needed, I'll bring the Shaman into the mix. Clerics, Bards, Warlords, and (presumably) Artificers all allow an ally to spend a surge + extra healing (in the form of D6's). Clerics grant the most extra healing (their Wis mod + the standard D6 progression), Warlords grant a baseline amount (standard D6 progression), and Bards provide a free slide and slightly more consistent healing than Warlords (Cha mod + standard D6 progression minus 1D6). All three of these classes can use Healing/Inspiring/Majestic Word to allow an ally that's in deep trouble to spend a surge and then grant them some extra healing. A Shaman's Healing Spirits, on the other hand, breaks the paradigm by providing spread out healing (the target can spend a surge and an ally adjacent to your Spirit Companion (SC) gets the extra healing, which is the standard D6 progression).
So why does this matter? For starters, it diminishes the power's use as an "emergency" heal because the ally in trouble isn't going to get any additional healing (from the base power; items and feats may add some) beyond their healing surge. Second, whether or not a second ally recieves extra healing is dependent on positioning, which may not be favorable during an emergency situation. Thus, Shamans are somewhat lacking in their ability to react effectively to emergencies if they rely too much on Healing Spirits. That's not to say that the power is garbage, because it does have its uses; namely, it's good for proactive spread healing. That is, you can let an injured character with surges to spare spend one, and let another ally with few surges get some bonus healing, mitigating their need to spend more surges than they have to. Note that the Bear Shaman's spirit boon does grant some extra healing (to both recipients of Healing Spirits, to boot), giving it more utility as an emergency heal.
In addition to the other tactical nuances that the Shaman exhibits, they have to worry about having a "weird" 2/enc healing power. This further reinforces what a complex class the Shaman is, and also affects how it interacts with the party. As I stated before, a low surge striker like a Rogue or Ranger would probably love having a Shaman in the party, provided that he (or another party member) had access to at least one other good emergency heal. Parties that lack characters who can easily get themselves out of trouble might not be the best fit for a Shaman. Overall, due to their spread out healing Shamans are good at keeping the HP of the whole party fairly stable, but if things go south they may not be able to deal with it as effectively as another leader class. It could be argued that a Shaman might make it less likely that an encounter will go awry by keeping everyone fairly refreshed, but regardless emergency situations will inevitably occur, and these are the times when the PC's lives are most on the line.
Just something for Shaman players to think about I guess. The thought occured to me when I was playtesting my World Speaker Shaman for the first time, and in a relatively easy encounter (level = party) I blew both of my Healing Spirits (to give the Rogue some surge-free bonus healing both times). I'd been running a Cunning Bard with this party before, and usually with an encounter of this difficulty I would have maybe used 1 Majestic Word.