It does this for SCH/SMN and the new jobs implemented after ARR, but it doesn't do this for the base game jobs and frankly it's just REALLY annoying when I'm leveling jobs that aren't my mains. Why not just make this a blanket thing that all jobs get the benefit of now that we're several years into the game and cross-class job abilities are no longer a thing?
It's not possible though as the base classes do not require the Soul Crystal to be equipped, only the weapon. The 'classless' Jobs rolled that together so that's why they equip the Soul Crystal automatically when you equip it's weapon. But base classes start without the Soul Crystal so hence why just equipping the weapon does not change it to a Job, you need to equip it seperately, which takes, what, two seconds extra tops? It's hardly an issue, if you need to to change to another Job just set up a gear set with the Soul Crystal equipped for that class and you will be able to change it automatically.
And as for why they don't roll the classes into Jobs automatically is because the classes were a legacy of the original FFXIV (version 1.0), which players who created characters in 1.0 can transfer their 1.0 character data into ARR with all their EXP, levels, classes and Jobs intact (along with all their existing items and gear). So the game has to keep the 'base' classes and Jobs seperate to avoid bugs with transferring in 1.0 data. It's a legacy of the original game that cannot be changed.
I'm confused, I've not had to re-equip a job crystal for like... 5 years. Unless you're equipping weapons that don't require a job crystal by which point I ask why you're equipping solely weapons in order to change job and not employing the use of gearsets.
I'm not sure what the issue is here, once you unlock a job you add a new gear set for that job and when you switch to that set the job crystal is automatically equipped.
Also at that time delete your gear set for the class, you will never need it again.