you get more Additional slots for actions without having a job soul crystal equipped. and if your level is synced to to Lv29 or below. it stands to reason any benefit from your job would be negated. (except SCH) so if you really want to... you could just take off your crystal and use more additional actions. also there some cross-class actions that are usable by your class. but not by your job. i suppose it all depends on your play style. but there ARE reasons to un-equip your jobs soul crystal.