The idea of support classes surely is nice but they are way more of a headache than they are worth in practice.

Once they design encounters with their support in mind they become mandatory for every raid, similar to WoWs Sunwell back in the day.
Not enough Shadows / Shamans online? Well cancel the raid.

As far as a broken trinity goes: Guild Wars 2 tried that and the content ended up being a horrible zerg fest. No thanks.