It very well might work like that, but keep in mind that such a mechanic has to be universal. It may make intuitive sense that only the Lancer Class can have the Dragoon Job, but then the Development Team has to apply that rule universally across all the Jobs.
- Only [X Class] can have [Y Job].
- Only the Lancer Class can have the Dragoon Job (makes intuitive sense).
- Only the Archer Class can have the Ranger Job (also makes sense).
- Only the Pugilist Class can have the Monk Job (makes sense too).
- Only the Marauder Class can have the Warrior Job (makes sense).
- Only the Gladiator Class can have the Thief Job (the only DoW that can use a shield is also the only one that can be a Thief?? Not very intuitive).
- Only the Conjurer Class can have the White Mage Job (debatable, but OK).
- Only the Thaumaturge Class can have the Black Mage Job (so Black Mages would have to cross-class conjury elemental nukes at reduced effectiveness, since Conjurers won't be allowed to be Black Mages. That doesn't seem to make much sense).
- Only the ?? class can have the Red Mage Job (ran out of classes).
- Only the ?? class can have the Paladin Job (ran out of classes).
- Only the ?? class can have the Dark Knight Job (ran out of classes).
See the problem. Now one way out of this problem is instead of making classes to jobs one-to-one, make them two-to-one.
- Only Lancers and Marauders can have the Dragoon Job.
- Only Archers and ?? can have the Ranger Job.
- Only Pugilists and ?? can have the Monk Job.
- Only Marauders and Gladiators can have the Warrior Job.
- Only Gladiators and Pugilists can have the Thief Job.
- Only Conjurers and Thaumaturges can have the White Mage Job.
- Only Thaumaturges and Conjurers can have the Black Mage Job.
- Only Conjurers and Gladiators can have the Red Mage Job.
- Only Gladiators and Conjurers can have the Paladin Job.
- Only Marauders and Thaumaturges can have the Dark Knight Job.
New problem. All the Jobs now have classes, but because there are 10 Jobs to 7 Classes we have an imbalance. The only fix for this imbalance would be to add 4 additional Jobs beyond the ones listed above. Doesn't sound bad, right??
With 14 Jobs to 7 Classes things are balanced and the game is ready for launch! YAY!!
However the case, to maintain that balance, any time a new class or job is introduced in the game a corresponding number of classes and/or jobs MUST be introduced in order to maintain the balance. So if the Development Team wants to give us a new Class, they have to come up with two new Jobs for that Class. If they want to give us a new Job, they have come up with a new Class and an additional Job to maintain the balance. This is in addition to balancing all the skills and abilities of the new Jobs and/or Classes. Seems like a lot of extra work just to keep people from being axe-wielding White Mages.
---OR---
The Development Team can have no restrictions and let any Class play as any Job (mind you I'm not talking about unlocking Jobs, I'm talking about playing them). Then players (individually and as a community) will have a chance to experiment and decide what they want to play, both solo and in a party. Also this will allow the Development Team to concentrate on balancing skills instead of the number of specializations a particular Class may or may not have access to when introducing new Classes and/or Jobs.
So I understand where people are coming from when they say that only certain classes should only be certain jobs. But enforcing that in game, with code, brings its own set of complications.