The makers of MMO games have been competing with each other for over a decade in a bid to get players of healing characters to base ourselves in their games.
Early internet graphical games generally did not have healers. In Doom and Quake it was assumed that the fun was killing people, you got a medpack if you were lucky. In Diablo 2 there was an embryonic healing build in the Paladin class but it was very underpowered.
However MMOs adopted the principle of allowing some characters to heal in return for doing less damage. This is based on two types of player:
1) Some players just want to win. They would wash virtual windows if it got raid bosses killed. They will adopt whatever will provide the most powerful boost to the team.
2) Some players just want to frag. This is traditional in games, in single player RPGs you wouldn't get very far if you couldn't kill monsters.
When you combine 1) and 2) you enhance both of their gaming experiences. The guys who just want to frag die less and get more frags per death. This makes them feel more powerful which they like. The power-gamers cause a team to be more effective which they like.
Once you have this then you can add the (optional) third pillar of the trinity: the tank. This is the guy controlling the fight and getting all the love from the healers, it's a popular job.
There's a fourth optional job: crowd control.
The shift from Everyone is dps to the Holy Trinity created an economy in which the currency is types of player.
DPS are currently ubiquitous, everyone likes fragging, everyone who plays these games played solo games as a kid where they fragged.
Healers, are perceived as the good in short supply and high demand in this economy.
Tanks, until recently were assumed to follow the healers which makes sense as they are dependent on them. However the recent move by Blizzard to specifically reward the least populated role is seen as an incentive for tanks.
This has led to game development companies playing the Great Game of Capture the Healers.
If you have a moderately successful MMO and I release a competitor that draws your healers away, that's really all I need to do. Because everyone else will follow them. No one wants to play a Trinity game as a dps or tank where it's difficult to find a healer.
This started with Everquest. In the predecessor game, Ultima Online anyone could attack anyone else. This meant that healers were usually victims for passing killers, characters built to frag.
Everquest poached most of UO's healers simply by making it less easy for them to get ganked.
Warcraft designed its healers around the principle that each would have a Fragmaster option. So Priests could be Shadow Priests, Druids could turn into lions and bite people, Shamans had two powerful dps options and Paladins had a holy warrior mode. However at max level things didn't quite work out as advertised, generally level 60 hybrids who wanted to dps in raids were laughed at for being noobs.
This opened an opportunity for Vanguard. As it turned out Vanguard was dreadful and got almost everything wrong but it did have extremely attractive healing classes. There was a martial artist who healed by beating people up, vampiric blood mages, and so on. The threat of Vanguard prompted WoW to rethink and so their healing classes became genuine hybrids in The Burning Crusade expansion, with Druids being the popular tanks at the start and Paladins the most popular tank by the end. This upset a lot of warriors but they don't matter too much because they're not healers (although this may have sown the seeds of WoW's current tank crisis).
Two new contenders emerged in 2008. Both tried to steal WoW's healers by making more aggressive versions. Age of Conan had Bear Shamans, who had to beat people up in melee to heal effectively and Warhammer Online had a number of classes who had to use some dps to best heal. For instance, Goblin Shamans have two gods and you make one happy by healing one happy be dpsing, each mode buffing the other mode. They also made healers attractive by making them overpowered. Tempests of Set were flavour of the month when Conan launched, Disciple of Khaine and Warrior Priests were very strong in Warhammer.
WoW again proved equal to the challenge, mainly due to other factors.
In 2011 we have Rift which has designed healing into 3 out of the 4 classes. In fact everyone is a hybrid in Rift and the game is designed to be incredibly flexible for hybrids. For example I can be healing you, then if I see I'm not really needed I can run out of combat, click a button, and come back as a dpser. They still have different gear at end games for different roles, but I think this was an oversight, a feature they inherited from older games and should have questioned but didn't.
Now I'm not saying that capturing the healers means you capture the market. Vanguard showed that a game can have very attractive healing classes but still fail for other reasons.
But it is a part of the Great Game, the only game worth playing for designers, of winning the market.
And because of this we will see more design aimed at making life attractive for hybrids in the future, we will never see pure healbot classes again and we will, in time, see gear homogenised so you don't need two sets to do two roles.