The reason jobs were implimented was because everyone had the same cross class abilities (and you could cross class anything)
So they picked the best abilities from each class, they all jumped on ARC for the range, and later all jumped on LNC when ARC was nerfed. (LNC had more range and could avoid AoEs other melee couldnt.)
Everyone had Sentinel for the mitigation on Ifrits Hell Fire., Everyone took up (i believe it was Berserk) w/e was the best offensive CDs at the time, and some took up Defender2. Also for mitigation.
The only difference between your class and another, was how much range u could reach with your class.
They nerfed armor, to make ranged classes stop trying to wear plate, even though it had already not been that great for ranged characters.
They nerfed ARC to stop full parties of ARC.
They nerfed DoH/DoL classes, to stop stun locking bosses with DoL/DoH classes.
They added Lv15, 30, and 45 abilities that were class speciffic, to try and give each class something unique(I honestly LOVED this system, was my favorite of them all)
but people argued the core abilities are the cross class abilities, and they were still too similar.
(Not like any of that changed by adding jobs really. Just the illusion of difference)