Having Disc have a mandatory spot may not have been a problem back in 25H raids, when you had 5-6 healing spots to work with, but it's absolutely a problem in the 20 Mythic setup. It's already been proven this tier that while they give lip service to the "5 healers is standard" concept, for all practical purposes, almost all fights are 4 healer fights. I think that Imperator and Blast Furnace (and maybe Beast Lord the first week before the nerf) were the only fights this tier that were typically 5 healed.
When you only have 4 healing spots, and 6 healing specs, it is absolutely a problem for one of those spots to be reserved for one specific spec, because you then only have 3 spots remaining for 5 other specs combined. That essentially gives Disc 25% representation, with every other spec fighting for an average of 15% representation, and makes that one healing spec effectively 40% overrepresented. On top of that, the practical reality is that at least one Holy Paladin is also all but mandatory, making 50% of the spots locked to 2 healing specs with 50% remaining for the remaining 4 healers. That is just not acceptable. Either they need to go back to needing 5-6 healers for every fight (in which case mandatory specs is somewhat tolerable), or they need to balance the viability of the throughput healers so they have things in their toolkits that are equally as required as that of Paladins and Disc.
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Overall Mythic parses are heavily skewed by the early bosses and by very casual guilds that are just getting into early Mythics. Lower ranked guilds generally can't tailor their raid comp; they have what they have, and what they have is heavily infleuenced by player population (i.e. Resto Druids are the most highly represented healer in the overall player population). For example, there is something like 7 times as many Beast Lord parses as Blast Furnace. If you parse out only the last bosses - IM, BF and BH, you get the following representation numbers:
Disc Priest - 25.7%
Holy Paladin - 25.1%
Resto Shaman - 17.0%
Resto Druid - 16.5%
Mistweaver - 10.5%
Holy Priest - 5.7%
The numbers for the actual end tier bosses very closely mirror the paradigm of - Disc and Pally have 50% of the raid spots locked up, other 4 specs get to fight for the scraps problem I was referring to.
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This site gives another view of the issue - it shows you the percentage of the overall player pool with 1 or more Mythic kills in WoD play each spec.
http://www.worldofwargraphs.com/statspve-classesrepartition-0-0-0-0-0-0-0-0.html
Looking at the healing specs:
Resto Druid - 5.1%
Disc Priest - 4.3%
Holy Paladin - 3.8%
Resto Shaman - 3.5%
Mistweaver Monk - 2.0%
Holy Priest - 1.5%
So, when you look at the raiding population (or at least the raiding population with any level of Mythic experience), Resto Druids are the most played healer by nearly a 20% margin. However, when you look at healer representation at the highest level (i.e. 3 most difficult bosses), Resto Druids drop to 4th out of 6 healing specs. I think that speaks pretty clearly to a problem with the spec actively lacking viability at the high end Mythic level.