I'm curious to see if anyone else out there agrees with my observations/standpoints on this topic. Obviously, the below post is based on my experience with my guild during Mythic progression.
Back a few years ago (Sorry, I can't find the post), blizzard mentioned that they were moving away from encounters that required more than 2 tanks, due to the need to have a 3rd off-tank ready for raiding, because they didn't think it was a good decision to vary the amounts of tanks needed by fight.
In Nighthold Mythic, the healing requirements vary so widely from fight to fight that it's kind of ridiculous. 5 healing Spellblade, to 2/3 healing Augur is a wide, crazy margin of healing requirements, and the very difficult dps checks make it a requirement to bring as few healers as possible to make fights easier.
We're encountering situations where during progression, extra healers are not asked to go dps (because our dps is so much lower thanks to having to prioritize healing artifact weapon, gear, and trinkets) and instead are asked to sit out so they can bring in strong dps to push progress.
How many people out there think that this is a bad thing? Because we are so incredibly locked to dumping our AP into a primary weapon for healing, it severely hinders dps for those of us required to change specs to see content/bosses. Either that, or we have to play uber AP farm to keep 2 weapons up to par. I love healing, and the fact that it's "dps or gimp your raid" on quite a few bosses makes me quite sad.