Don't Wanna Sound too much like an Arse... But That's the Point of them at the Moment. The Jobs are Specialized Additions ontop of the Existing Training you've got with that Skill-set/Class. They ARE Similar because they use the Same Basic Skills as a Leap into Further power. Only one that Stands Slightly Apart is Summoner/Scholar, And Even they use the Same Basic Knowledge (Summoner is using the Knowledge as a "Lens" for much Older Knowledge which is Largely Lost).
Any Additional Jobs for the Basic Classes should use the existing Training as a Stepping Stone too greater power, Gladiators Adding "Dark Magic" too their Skills too boost their Power would be the Foundation of say the Dark Knight, Conjurer's Focusing Less on the Balance of Nature and more on it's Fury could be Geomancers Basis.
Not Gonna Argue.. The Job System could do with Some Tweaks.. But Breaking Everything away Completly from the Classes will make the Game lose some of it's Flavour. (Too me Anyway)