Originally Posted by
Colorful
As of now we only have to level classes that SE wants us to, open the gates to all possible abilities (if I'm reading your suggestion right) and you'll have things like everyone to get swiftcast + enhanced raise (not sure if enhanced abilities can be cross classed, but w/e) for battle raising easier... which would present balance issues, along with countless other abilities that may be useful for other Jobs. SE would have to start developing content around this.
I'd much prefer they ditch the cross class system and instead introduce a specialization and talent system, similar to WoW before they revamped it.