theres a reason why its called the holy trinity, let me explain some things, when you add a "role", you take account of the content to have that, i.e bosses need a tank, healer and damage dealers, and mechanics are designed to have those, with support, then the bosses need to take account of that role, otherwise why have the role right?
now, saying that, how many tanks and healers are there compared to damage dealers already?, what was that, the combined amount of main spec tanks and healers still being less then damage dealers?, add another role into that fragmentation, its a headache from the playerbase front, and the devs know this, they know this as they aquire data in game, adding another role thats needed in a fight just makes things even harder to do from a logistical standpoint and waiting times are longer