Gamasutra - Class Design
Hybrid class design article discussing the value of said roles.
The hybrid issue is exacerbated by the fact that MMOs are both solo and group games. If people only played MMOs in groups, a character able to soak a lot of damage but deal no damage would be viable because the other people in the group could deal damage for them. The individual character could be one dimensional (a pure tank) because the other group members fill out the other two parts of the trinity (DPS and healing).
However, studies have shown that even in group-focused games, players spend a lot of their playing time doing things on their own. Even if a character is the best healer in the world, if they can't take or dish out at least some damage they won't be able to operate outside of a group. Soloing requires that the character be able to deal damage, plus the ability to absorb, avoid, or heal the damage taken.
Therefore, once the design decision has been made that every character should be able to solo -- a decision that has been made practically mandatory by the successful example of World of Warcraft -- it automatically follows that every character must be a hybrid and therefore subject to the paradoxes of hybrid design. This is a universal problem, not just one that affects certain classes within a game.