Warlord has predictable dmg, it's the same story as morchok, the ball bounces around. You don't need speed shields for shit. As disc you can line up PI+AA and a bunch of shields prior to black, as holy you can do what? Renew? Shield 1 target? Right...
Ultra, you take red as disc and just spam smite till shit gets tough (attonement works with red btw!), then you spam PoH. I'll take 2-3mill dmg off the end any day over 10-15% more healing at the end.
Spine only requires 2 healers, as disc you can just attonement-nuke the amalgamations and assist if the other healers need help (with rolls or nukes etc.). As holy you just add death weight, as the healing becomes a joke.