The idea of a niche is a good one from some perspectives, you need to take a Rogue because you need the debuff poison he has, similarly you need to take those 2 priests for the Mind Control they offer for that boss fight. This structure however falls down when you consider that this means that there are effectively fewer raid slots available for other people, niches, while defining us, lead to a reduced ability to do what we want to do.
I can look at tanking, and see that there used to be niches, Bears were off-tanks, Paladins AoE tanks, and Warriors were the eternal main-tank, in short we were forced into roles due to mechanics, options and talents. This failed in many ways, those upstart Paladins were main-tanking bosses, and Druids were only held back by the crushing blow mechanic, tanks didn't want to be niched. It can be seen in reverse as well, while Warriors loved the almost guaranteed main-tank slot, they hated the effect in 5 and 10 man instances where they were sidelined by Paladins ability to AoE tank effortlessly (well not really, but Consecration basically made up for very bad DPS targeting / control). With the changes in WotLK we see the reverse, now we have Warriors worrying for the loss of their main-tank role due to a lack of "utility" despite holding the tanking utility crown still, we see Paladins happy to lose the AoE role (mostly) provided it brings their threat, mitigation and utility in line for tanking with Warriors.
There are problems with this of course, we see it in the Hybrid vs Pure debates, we see Rogues worrying that DPS warriors offer as much DPS, and those same DPS Warriors worrying that Retribution Paladin will be brought instead due to the ability to hearth, respec, change gear and then heal. People came to depend on their niches, 2 rogues in a raid was fairly common because it made good use of the melee group, 2 Rogues, a Warrior, an Enhancement Shaman and a crit-Bear. Similarly Shadow Priests were shoved in caster groups, you wanted the Shadow Priest, Elemental Shaman and 3 Warlock group twice because it did good dps. People grew used to being a niche role, and they came to like it.
Removing niches hurts this, suddenly our damage has to be similar across the board, making player skill important (though the distribution of skill needs to change really, the complexity of maintaining a proper Affliction rotation compared to a Frost rotation is much different for very little gain if any), our tanking needs to be based off of the common tank skill set (which is the tank equivalent of dps), and the utility in a raid assignable mostly through bringing a small number of classes to get the buffs. The one role we haven't really de-niched is in fact healers, leaving a deficit.
We see Discipline Priests and Paladins as single target healers, yet the other three healing specs are capable of near equal single target healing (otherwise 5 man instances would be an eternal pain for them), yet we do not see the reciprocal, a Paladin or Disc Priest cannot AoE heal to anywhere near the effectiveness of the other classes. This breakdown is bad, it means that there are two solutions, if the raid damage is like pre-BC we see Paladins and Disc Priests in too many numbers shunning the rest, or if we see TBC type raiding we see Shamans, and CoH Priests too often. Healers, like tanks, need to have their core skill-set homogenised a little, giving all healers access to single target, AoE, reactive and shielding skills, then making the differentiation in how effective and synergistic these are. I don't want to see the day when we say:
"Ok, Paladins on raid healing and Shamans tank healing"
That tends to go against the theory of the classes, Paladins and Disc Priests will and should be the strongest (by a margin of 20-50%?) single target healers, it should be their effortless role, similarly a CoH Priest and a Shaman should effortlessly heal raids and groups (by a margin of 20-50%?), leaving Druids as the main passive healer class, rolling lifeblooms, HoTs, reactive healing spells out onto the targets, suddenly classes are all capable, but have roles to fill. We can even differentiate within a role, a Paladin is a solid throughput healer, being able to throw large heals quickly and infinitely onto a single target, or throwing smaller splash heals around, the Discipline Priest meanwhile works more through shields, preventing damage, redirecting it and smoothing the incoming damage curve.
We can see CoH Priests having powerful, large scale heals (hitting 5-10 people at once for fast, reactive healing), Shamans offer more targeted healing through Chain Heal as a more powerful solution to raid damage.
The key to making it work though is synergy, two CoH Priests should be less effective in the general instance than a CoH Priest and a Shaman doing raid healing, that is to say that the damage rate / type / spread should favour stabilising people quickly (the benefit of CoH), while allowing a focused healing through Chain Heal. This can be achieved by moderating damage or through buff and debuff types, perhaps CoH lands a "drained" debuff on its targets, increasing healing done on them, but reducing further CoH healing, or perhaps doing other some other targeted buff. Smart heals are useful here, but should be less powerful than a targeted heal in terms of effectiveness.
I won't say we don't want niches, but what needs to happen is for niches to be complementary and to allow almost any combination of classes to complete a raid given sufficiently good players, its not to say that not stacking 1 Shaman, 1 Priest, 1 Paladin and 1 Druid isn't the best base combination, but that 2 Priests and 2 Paladins can still get through it if they make no mistakes, its the ZA 4 Timed Chest run, in T4 gear, sure its easy in T6, but if you make no mistakes, have the skill and 9 other people that don't make mistakes and have perfect pathing then it can be done.
Role on homogenisation, because it can allow us to be more differentiated without leaving us stuck in a niche with no where to go.