While Gilthas puts on a good point, I wish to add onto the social aspect of such options.

Diversity is neigh on impossible to maintain in a game culture that worships gleaning efficiency and uniformity in every conceivable aspect.

Take the Subjob system. A Job was expected to have a specific set of sub classes it was to use, and woe onto the player that experimented beyond the experienced norm. Same went for equipment or weapon usage. The use of Red Mage alone was a perpetual debate on even the smallest of party trash mobs.

Here, the roles, the flexibility and usefulness are clearly defined. The game gives you limits and bounds with a bit of flexibility of use but primarily maintaining a clear focus. The trade off is that every player can still play every class on a single character- a level of diversity and depth that is still lacking in most MMOs to date - something I feel players overlook.