Edit: While I haven't tried equipping other weapons on all classes, it seems like it might only apply to the classes whose weapons have the little extra text under it specifying that it's for "Class only" even though there are several weapons that are class exclusive that don't have this text under them.
Kanna can equip other weapons, even though fans say "Kanna Only".
Jett can't equip other weapons, even though guns are used by multiple classes.
Granted, those are both non-KMS classes. But it does mean the message on the weapon and the restriction on equipping other weapons are not both generated from a single definition.
Interesting. I can't check right now but is Pathfinder able to equip other weapons?
The restriction might just be there to ensure that new classes must use a certain weapon, and thereby have to invest in enhancing and cubing one. Even if the skills wouldn't work with another bow for example. Sounds like unnecessary spaghetti to me.
While I agree that nobody is being forced to remake their equips to min-max when you can reach Black Mage without these scrolls, the X scrolls are already in Philo books. So yeah it is a new p2w upgrade option and will eventually devalue (market wise) items that are scrolled with any "lower tier" scrolls. Didn't primes start out this exact same way?