The practice you get from levelling a class from 1 to 50 should prepare you fairly well for its job counterpart since they have a very similar role in the party with minor alterations to their ability. If you do wish to practice with a job you have the option to join a party with more casual objectives such as farming keys where your sole performance will not greatly affect the party's success.

Although it may be disappointing if you have levelled your favourite classes already and will not be able to use their jobs as much as you like, remember that new content will always be introduced to give you more to do. Including a higher level cap - giving you the chance to level up as the job as well. Aside from that small issue I am glad the jobs and classes share the same level. I would rather not be forced to level a job after going through what will likely be an almost identical experience from when I levelled up the class.

I think partying as the job may be more enjoyable than as the class so I am waiting to level some classes until the jobs are released so I can level as the job instead.