Please do not lock Jobs to the Weapon Class that unlocks them.

[Reika]

Like i said earilier, your job may not be locked to your class, or vice versa. Class is still changed by equpping a different weapon, job stays the same.

The job system - that series-defining trademark - will be implemented in FINAL FANTASY XIV (details such as specific jobs TBA). Quests to unlock each job will become available once you have reached a certain rank in the corresponding class.
Rather than being advanced versions of classes, we would like players to think of jobs as specialized playing styles optimized for partying. While playing as a particular job will render you unable to equip certain actions of other classes, you will be compensated with access to exclusive actions, which can be acquired through quests.

In practice, players will find the existing classes effective when playing solo or in certain party setups. Jobs, however, will shine in party situations where specific skill sets are demanded. Now, you didn’t hear this from me, but there are also rumors circulating that quests for obtaining job-exclusive equipment are also in the pipes.
We at the development team are currently hard at work on the job system, and expect to have details to share with you soon.

And the above underline says what i said as well. you will lose access to certain class skills and/or spells, while gaining new ones specific to the job.