Group combat in World of Warcraft is a mix of damage dealing, aggro management, and healing, with different classes being specialized in those three aspects. But in soloing and PvP only damage dealing really counts. Most aggro management skills to increase or reduce threat don't do anything solo or in PvP. Healing is nice in solo and PvP, but not downright essential. Especially in soloing, healing doesn't get you anywhere, you need to deal damage to kill the mob to do the quest. So from the three different specializations, the damage dealers have obvious advantages: they can use the same talents, gear, spells and abilities in group combat, in solo combat, and in PvP. Meanwhile somebody who specialized in aggro management, like a tank, or in healing, is penalized. Either he has to collect several sets of gear and pay for respecs when he wants to move from grouping to soloing and PvP, or he has to live with being less effective in soloing than the damage dealing classes. Sometimes even both, there is simply no way that you can make lets say a paladin deal as much damage as a rogue or mage in similar quality gear.
Yesterday I was hanging out in the looking for group chat, before I found a guild group going to Magister's Terrace, the new dungeon. And the chat was full of half-complete groups shouting for a healer, a tank, or both. Even in guild chat we regularly have the same problem, healers and tanks are in short supply. Everybody wants a healer and tank to group with, but too few people want to actually play one themselves. For me that is a classical case of people voting with their feet: Everyone knows that by playing a healer or tank you are gimping yourself for solo and PvP, so they tend to rather play classes that are easier to solo.
What I think Blizzard should do is making playing a tank or healer more attractive. That would not only help the players who have a tank or healer, but it would over time also increase the number of tanks and healers in the game, and thus help everyone to find a group. And the key to encouraging people to play a healer or tank is certainly to improve their soloability. Making healing bonus count also as spell damage was a good move in that direction. But patch 2.4 was sadly devoid of any further improvements, especially for tanks.
And of course one frequently discusses point here is the cost and ease of respecs. Patch 2.4 slightly reduced the cost of respecs, you don't have to pay training costs repeatedly in addition to the respec cost. But respec'ing having a cost at all isn't affecting the different classes in the same way. If I would respec my mage from frost to fire, that would be purely for the fun of getting to use different spells. The mage would still be a damage dealer, and in most cases he could still use the same gear. For my protection warrior, a respec is something completely different: I basically switch from being a group character to a solo character. I need two sets of gear to be efficient in both forms, and whatever spec I choose, I become inefficient in the other mode. Nobody wants a fury spec warrior as a tank, they are just not good enough for the job, unless they are completely overgeared for that specific challenge. And the same is true for healers, groups don't want to rely on shadow priests or enhancement shamans for their healing. So why does Blizzard want to have a barrier preventing healers and tanks to switch easily between group and solo mode? It just discourages people from playing one of them in the first place.
Of course playing a healer or tank has one indirect reward: ease of finding a group. As a healer (not necessarily as tank) there is also a good chance that you'll get a raid spot easier than a dps class. But World of Warcraft is not a highly group-centric game, and lately the rewards for soloing (daily quests) and PvP have been significantly improved, while the rewards for grouping didn't get all that much better. And then a vicious cycle kicks in: People find it easier to do PvP, thus groups aren't needed that much, thus healers and tanks aren't needed as much, further reducing the appeal of those classes, making it even harder to find them for a group, which increases the appeal of PvP even more. Unless Blizzard wants to turn WoW into a game which is all about soloing and PvP, they should improve the appeal of playing a healer or tank, as long as there are still some of them left.