Beyond the trinity
The idea of the three primary archetypes (Healer, Tank, and DPS) is pervasive enough in the genre as to be referred to as the "holy trinity". However, it's not the only way to break down the possible commodities in an RPG. Some designers use six categories, others use only two (increasing damage out and decreasing damage in).
Some of the alternate categories are things like "buffers/debuffers" specialized in enhancing their allies or debilitating their opponents, and "crowd control" specialized in rendering opponents helpless by rooting them to the ground, freezing them in blocks of ice, etc.
The exact number of commodities isn't important in this analysis, as long as the commodities are not perfectly interchangeable. For example, if the DPS can kill a group of enemies before the enemies reach them, there's no real need for tanks or healers. If freezing an enemy in a block of ice can completely remove the need for tanking, then tanking and crowd control can be considered a single commodity and one class will emerge as the preferred vendor for that commodity.
The pressure towards specialization exists regardless of the number of archetypes used. A healing/crowd control hybrid will still have a comparative advantage in a single role unless one or more of the assumptions underlying this economic model is broken.
The standard model of MMO play fits the comparative advantage theory a little too well for comfort. Specialization is the natural course of the character economy, reducing game complexity and forcing players into a minimal number of tightly constrained roles. Broken hybrids are not an aberration in the class economy, they are the default.
However, this theory is not an inviolable law, but instead relies on a number of fundamental assumptions about the class economy being true for the theory to apply. Clever game design can break those assumptions, lessening or removing the pressure to specialize.
Existing MMOs with successful class economies are already breaking these assumptions, either deliberately or by accident. Hopefully, laying out some of the theory underlying these class dynamics will make hybrid design easier for future MMOs and indirectly make those MMOs more fun for their players.