Here’s a secret - it would be trivial for designers to code monster AI that did the smart thing every time. Kill the healers, then kill the DPS, from highest DPS or squishiest to tankiest. If someone casts crowd control, don’t let them cast it twice. Ignore damage output, and target troublesome classes. And above all, don’t divide your damage - have the whole group of enemies burn down one target. Yes, this strategy is effectively what PvP groups do in the arena.
The problem is: so you think its hard to find healers now? It becomes 10 times harder when the healer experience is looking at your corpse on screen for 90% of the fight (this is, incidentally, the experience of most newbie healers in the arena). Take healers out of the equation, and you move the problem to whichever DPS is squishiest. Which is to say, the problem with realistic AI is that it’s NOT FUN. And fun is still, you know, kinda the point.
Note: many other games in many other genres have AI that would be considered borderline retarded if a player played that way. This includes shooters, strategy games, RPGs, GTA, etc, etc, etc. Stealth games are the worst offenders - the guards stop looking for me if they don’t find me after 20 seconds? The role of AI is not to be realistic and not to crush the player, but instead to challenge the player and be ultimately a solvable puzzle. (I’ve written about this before: You don’t want realistic AI) The point is not that the trinity is the only way to solve this problem, but rather that giving ’smart’ AI to the mob that bypasses the meat shield is going to result more in frustration than in interest. Take out the trinity, and another ‘puzzle’ needs to take its place.