The point of the main job / sub job system was that the main job had a defined set of abilities that gradually got better / more defining. Having a singular sub job added enough in way of customization, without breaking the game. Players had to pick and choose what abilities would be best suited for certain events.

Example: /war was better at over all DoT. /thf was better at threat control.

This system was great. It made each class feel unique, and then made them feel even better by just switching how the role was played. I simply don't understand why people like being able to take every ability from every job to make one character. If we don't have to give anything up, then it isn't really customizing. It's just adding another toy to the box.