Do other schemes exist? Sure. The action-movie scheme is smart guy/big guy/face guy/wild card. (The A-Team! Exactly!) We could have wound up with something like that. There's nothing inevitable about tank/healer/DPS from player psychology or from the fantasy source material.
Here's my conclusion: We got the roles we got because Gary Gygax gave D&D magic-users d4s for Hit Dice.
In other words, the class role scheme emerged from the mechanics of D&D itself, not from the inclinations of the players or the source material. And the scheme was omnipresent when D&D-lovin' designers were working on the first MMORPGs, so they followed suit.
So You Wanna Be a Magic User
Let's look at original D&D and 1st edition D&D. This will be old hat for the D&D grognard, but if you weren't playing D&D in the late '70s and early '80s, you'll be amazed at how cruelly fragile the D&D wizard was.
The magic user class--what we'd call wizards nowadays--had four-sided dice for Hit Dice. Period. That means many of them should be running around at 1st level with 2 or 3 hit points. And this was back in the day when most players rolled dice (often 3d6 or 4d6-drop-the-lowest) for their ability scores. You had to have a really good Constitution to eke out another hit point or two; it wasn't like 3rd edition where a 12 Con is worth another hit point and a 14 Con is worth +2 hp.
Contrast those hit points--somewhere in the 2 hp to maaaaybe 5 hp range--with the damage from a single monster attack: 1d6 damage was typical, with some attacks greater or lower. Even the weakest pit trap dealt 1d6 damage. Get hit once, you might be dead. Get hit twice, you're almost certainly dead.
It gets worse. There was no negative hit points or unconscious state. Zero hit points means you're full-on dead. Monty-Python-parrot dead. Resurrection magic was a high-level affair, prone to failure, and you came back weaker each time.
But it's worse than that. No armor for you, either, Mr. Magic User. You had an Armor Class of 10 (the worst possible), and at 1st level you were unlikely to improve it with magic or a high Dexterity (because remember, you rolled those ability scores, and nothing short of a 16 Dex could get you an Armor Class improvement).
You know what? It gets still worse. Your saving throws--rolls to avoid various environmental and magical effects--weren't very good. You had few weapons to attack with, and your magic was the fantasy equivalent of a single hand grenade...if you were lucky and got a top-drawer offensive spell like sleep.
So go ahead, send that guy with 3 hp and AC 10 into the Caves of Chaos. He can't survive. Sooner or later, there's going to be an arrow with his name on it. It's the tyranny of math.
If you played a 1st-level magic user in the early 1980s, you could not survive enough encounters to reach 2nd level. The odds against are astronomical. If you did survive, here's why:
• Your table adopted house rules to be less cruel to magic-users specifically or less lethal in general. (Like positing a near-death state at 0 hp, giving magic-users more or better spell choices at the outset, and so on.)
• You were cheating.
• Your DM was cheating on your behalf. If he did a good job, you never noticed.
Now, the word "cheating," especially in the latter case, is more pejorative than I intend it to be. D&D is a cooperative game, after all, and whatever "cheating" occurs is often a victimless crime. Frankly, a DM fudging dice rolls to keep a 1st-level magic-user alive is an act of friendship.
The fragility of those magic-users, viewed through the lens of future D&D editions and computer games, seems to be an utterly baffling, counterproductive, and just plain bad design decision. Why on earth would Gygax set up an entire class to fail? I mean, anyone looking at 4 hp, AC 10 knows that it doesn't add up, right?
But Gygax was on the frontier of a whole new type of game, and so he was operating with a different mindset than most game designers. I believe that when he was building the magic-user, he was in simulation mode.
• Magic users had 1d4 hit points because they _should_ be scrawnier than thieves and clerics, and even scrawnier than a hobgoblin or an orc.
• Magic users had AC 10 because that's what ordinary peasants in tunics had, and there's no simulation reason for the game to treat magic users any better.
• You got one spell per day because Gygax figured that if Turjan (from the Dying Earth) could keep four spells in his head at once, then a beginner like you gets one spell in your head at a time.
Viewed through the simulation lens, all reasonable choices. It's only when you take those 3 hp into a dungeon that it falls apart.
Sherman, Set the Wayback Machine for 1982
Now imagine yourself in a basement starting your first AD&D campaign back in the early 1980s. Van Halen's Fair Warning is on the turntable and you've got your parachute pants on. Everyone is playing D&D by the book, and because 1st edition AD&D did a good job of obscuring the system numbers, nobody realizes that the magic user is doomed. The inevitable goblin arrow kills the magic user, and a pall falls over the table.
At this point, a couple of things probably happen. First, the guy playing the magic user probably rolls up another magic user or maybe an illusionist (same thing, pretty much). But the DM has better access to the game's underlying numbers, and it's probably dawning on him that the next magic user isn't going to be any more viable than the last one.
If the DM is really with it, he comes up with a house rule on the spot. But more likely, he starts cheating behind the screen to keep the new magic user from dying in the very next encounter. (Nothing wrong with that--if you're hanging out with your friends in the basement, you might as well have fun, right?) But the players don't know that the fix is in. Even the DM might not realize how far he's going to go to keep that magic user alive.
Independent of the DM, the players (probably subconsciously) change their tactics to keep the magic user alive. They're doing so not because doing so makes tactical sense. They're doing it just to keep their buddy Bob from getting bummed out because his D&D character died.
The other characters start standing in front of the magic user...and thus the traditional front-rank/back-rank tactic is born. More interestingly, the other characters start doing everything they can (attacks, verbal insults, silly/weird antics) to keep the monsters from focusing their attention on the magic user...and thus aggro management is born. If magic users were as durable as, say, the D&D thieves, I don't think either behavior would have become so pronounced.
So I think that's where our tanks came from. I need to make lunch for my kids, so expect part 3.1 soon, in which I'll explain that the healers came out of the same place--the early D&D game mechanics and Gary Gygax's urge to simulate.
Out of Context: "There's still more beautifulness!"
Music: Brad, Shame