Relevelling jobs makes little sense, because they are simply extensions of the current classes in various directions. A paladin will use many of the same skills as a Gladiator, with a few more gained via quests.

The reason they won't seperate classes from jobs (not only because of the above reason, that they're pretty much the same thing) is because they want everyone to be able to solo in the way they want to. This means that a conjurer will be able to solo relatively easily up to the max rank at the time, and then also switch to White mage when the situation calls for it but not have to suffer the shitty levelling process as a WHM who has little no offensive capability.

I don't get why people want to level it all over again. The difference between a Gladiator and a paladin won't be enough to warrant the need to relevel it in order to be 'skilled' at it. MMOs aren't that hard.

Classes meet the needs of people wanting to be casual and generally mould a class how they want to. Jobs meet the needs of people wanting classic job roles, and to feel like they're fulfilling a classic need.

I don't know why people are so hung up on it, for a FFXI reference: Imagine if a Scholar had to level up to max on the basic class, then the Dark Arts form and Light arts form. It's the same sort of situation, just extensions of a basic form.