I think it was how vehement he was with it...If your game is really open, and you have healers, a trinity will probably arise naturally. It might not be the ONLY method, your game might support kiting, walling and various other strategies that bypass the trinity. But saying "NO HEALERS WARBAGABLE!" made it seem like their only plan to end the trinity was to not allow healing classes, which is really dumb.
That and, as said earlier, it sound more personal and less rational than it should have. Some guy had his cleric GF cheat on him and leave with the main tank, and now he hates healers, is what it feels like.