Fluid hybrids cannot be equal power to single purpose classes, for game balance. In raids, underpowered classes or specs would (and are) shunned. Balanced fluid hybrids would never see endgame except in limited circumstances. Making hybrids modal allows them to be brought up to similar or equal power as a single purpose class without disturbing game balance too much, thus allowing them a place in endgame.
Here's the problem with this statement: being a hybrid (regardless of type) is in itself an advantage. If a modal hybrid was as good as a pure class, why would you bring the pure class? And if the pure class is better, why would you bring a hybrid?
If the hybrid is fluid, the fluidity is the reason you bring the hybrid. Sure the paladin may be a worse tank than the warrior, but she can off-tank an add, and after the add is killed, can help heal the warrior through the enrage. Fluid hybrids allow a raid group to adapt to multiple stage fights better and more smoothly because the hybrids can take a different role during each stage.
But if the hybrid is modal, she is a tank for the entire fight, and after the add is killed, becomes pretty much useless. In this case, why take the hybrid tank at all? Why not just take another warrior?
WoW sidesteps this problem a little bit by re-defining the pure classes (warrior, priests) as hybrids, thus allowing them to drag the other hybrid power levels up. But there is still a lot of sentiment that "priests should be the best healers". But if this was the case, why would you bring paladins? If priests are the best healers, modal healing paladins cannot tank, cannot dps, and would have worse healing than a priest.
- Modal hybrids must be equal to pure classes at the given role, else the pure class will be preferred over the hybrid for that role.
- But if they are equal at the given role, the hybrid is better because it is a hybrid, and offers additional options for the player.
This is a paradox. Blizzard has resolved it by redefining all tanking and healing classes as hybrids. Paladin healing can be as good as priest healing, because priests are modal hybrids and can become DPS.
A fluid hybrid does not need to be as good as the pure class. But they do need to be good enough to perform their part. And that is where historically that WoW has failed. Allowing hybrids to be good enough to, say, off-tank a mob while still retaining their fluidity to help heal after the mob is dead.
And the reason this is so, as most people pointed out in the previous post is because of gear.