Ding! We have a winner.
MMORPGs that have classes have tried to have other archetypes (and in games where there are "skills", players end up CREATING the archetypes out of the skills, so it becomes functionally similar. In the end, it doesn't work out, because unless those classes are useful, and useful often, there's going to be complaints from both sides:
1. The people playing that class complain that they can't get a group a lot of the time because most of the content doesn't require them to be there (why take along someone who can debuff the enemies if for all the battles you're fighting this time, you can kill them without those debuffs? Just take along another DPS).
or
2. The people that do the 95+% of the content that doesn't require those classes complain that they have to go and find someone of that class for that ONE moment when they are useful.
If there are hybrid classes out there that are only 50% as good as a Warrior tank and 50% as good as a Cleric healer, no one, at high levels, will want to use them for either role, and the people playing those classes, who may have chosen them early on because they sounded neat, end up feeling robbed when they get to high levels and realize no one finds them useful (EverQuest tried this with the ShadowKnight and Paladin, and had to buff them both significantly). If there's a class out there that has a special buff that's great for a few boss battles, but isn't necessary in most other cases, and it means they're a 25% less effective healer than the other healing classes, no one will want them except during those boss battles, and even then, they'll just take along one. EverQuest started with the intention that the Shaman, the Druid, and the Cleric were healing classes, but the Cleric was clearly better - guess what happened? A large group might have one Shaman, for slowing down the attack speed of the enemies, but had to have a large number of Clerics, you know, to do the REAL healing. Solution - the healing ability of the other classes was buffed substantially until they were nearly equivalent.
And let's not get started on the Enchanter, a class that for crowd control could be amazing, but in many mundane encounters with no need for crowd control, was used for Clarity and little else. Solution? Give 'em more ways to do damage.
In World of Warcraft, the new "random group" ability lets practically anyone join a group that the game puts together as "Tank, Healer, 3 DPS". In the game, in "standard" dungeons, the effectiveness of the tank, healer, etc. in those groups is determined more by their gear (and their individual skills) and less by which class they happen to be. Replace a DK DPS with a Hunter DPS in your average dungeon and assuming similar gear you'll end up with similar DPS.
In raids, sure, it's good, often essential, to have a mix (for example, when Onyxia is flying, you need ranged DPS to be able to, you know, hit her). But if there was a 4th archetype there, right now they wouldn't be needed. Any game would have to be designed from the group up with that 4th archetype in mind as one that is integral to the game. Right now, it's hard to envision what that archetype might be.