I'm not a priest. I do care about the effectiveness of my raid's healing corps. In my opinion, the raid healers should be choosen to minimize their number and maximize their effectiveness so that as many DPSers can be staffed as possible. The more DPS you can bring the faster the fight is over.

If they took that stupid Instructor Razuvious fight out of the game I'd only staff a raid with a single priest for fort. And even then it would only be a shadow priest because their dps is reasonable.

You aren't wrong, and the priest QQ hasn't gone unheard. Those of us who are picking healers for our raids know you are weak. Skill is always more important than the class though. You'd get a spot if you could pull your weight, but the deck is stacked against you. You have to be better than the other healers because your class is not strong.

Healing Priests are a jack of all trades, master of none class. Shamans are now decent at tank healing, and good at AoE healing. Paladins are excellent at tank healing, and decent at raid healing when beacon is sufficient to keep the tank up (many encounters). Druids are also jack of all trades healers, but they are also masters of HoTs. A druid can heal anything, but they also bring unique heals that no other class can provide.

Holy priests are meh. They bring nothing that someone else couldn't provide. They only get a spot if the player is exceptionally good.

Discy priests at least bring some utility to the table that is helpful in specific encounters, but the current content doesn't require them. If loatheb was a harder fight, a discy priest would have a guaranteed spot.