Starting a raid can be difficult, if indeed it is possible you will often end up with a very strange class balance or some kind of horrible, unintended flaw in the group make up. Typically this can be solved over time by recruitment and refining your raiding group, however doing so will often force you to move people onto a rotation, or worse lose someone you think of as a friend. For this reason Blizzards aim of homogenizing the classes makes a lot of sense, bringing together the class you NEED to build a raid into a smaller set allows raid leaders more freedom to choose what they want and who they want for their raid.
The idea of restricting the raiding game to a small subset of defined classes never really made sense, that is to say that making an obvious choice in terms of overall dps, tanking ability or healing effectiveness reduces the choices you have in game as the encounters must be balanced against the broadest skillset and highest dps or healing groups to make fights challenging for groups. What is worse is that this distinction is often set based in your choices at the character creation screen long before you ever knew your choice would make you superb or subpar in a part of the game days or weeks away. These choices and differences can be aptly demonstrated in the tanking, dps, and healing trees.
Taking Warriors on the Reliquary of Souls fight, phase 2 requires the tank to reflect the Deaden onto the boss otherwise the damage taken is nearly unmanageable. Two tanking classes cannot do this meaning that to progress they need to overgear the content.
The third phase of this fight us much superior in terms of balance, the rage / mana burn coupled with the scaling damage favours a paladin tank, however in no way rules out a warrior or Druid tank.
The second phase is a clear example of a character screen choice ruling out and making life in the raiding game much harder, by contrast the second shows a limited bias but in no way penalizes your choice. This difference is key, a minor benefit in a niche area or fight is not unbalanced provided the niche is not every fight, however if the tools are not there for every class to at least be ok at something then a problem exists.
Damage per Second:
This can be seen easily between warlocks and mages, mages have lower dps overall and lower survivability, trading it for utility and dispelling. This difference in dps leaves mages struggling for raiding spots because of their dps and instead being brought primarily got their buffs and utility. Again fights need to be balanced around the dps that the DPS classes can bring rather than making a fight trivializable using higher dps classes.
Warlocks simply being the better choice due to synergy and mechanics destroys the balance here, while mages offer something warlocks can't beyond that minimal number of mages to do it warlocks are strictly better choices. This is different from the tank case in that there is a ceiling on your class, this your competition is much higher and the class balance which should be even is instead skewed.
This can likely be best seen with Paladin healers in the end game where restoration shamans and circle of healing priests are simply a better choice due to the large aoe damage and smart heals beating out longer casting efficient heals. Again there is no niche because every other class can fulfil the main tank healer role, however the fundamental tank damage throughput limit means fight mechanics must use raid wide damage, where the smart and aoe heals become a better choice.
This is not a matter where skill can make up but rather a set of game mechanics that penalises a class for becoming a specialist in a role. Rather than becoming the best choice for that role they lose all roles because they cannot fulfil a different specialized role.
So my point is:
Moving away from these situations is a good thing, it moves towards the idea that a raid is composed of 10 or 25 people, broken down into tanks, healers, dps, and niche roles as opposed to a firm requirement for non-niche (such as a Mage tank on high king maulgar) class roles such as a shadow priest purely for their mana and health regeneration. At first glance this reduced the chances of a hybrid class getting raid slots and puts everyones chances down, however that is far from the truth.
By becoming competitive in a generic type role (tank, dps, healer) and providing a group benefit, specialists can shine in their niche roles but never worry that their choice at character creation was one which will penalize them. Spwcialisation has benefits but it should never stop you progressing or limit the slots available to you on an arbitrary basis.
--- a mobile post, forgive the formatting and spelling as the auto correct is painful.