Certain jobs have traditionally not been acceptable as video-game jobs. While the field is certainly diversifying (whoever said that a painter or a musician don't have the skills to save the world?) there are definitely roles that will never be involved in the quest to destroy Evil. Accountant (GP Rain?) and custodian (Aqua Rake?) are included amongst those who will never ever been join in your ragtag group of rebel questers. You can use this method to easily ascertain who will join your party at a later date.

Certain professions are also far more likely to join. Thieves, assasins, pirates, rogues, and other such scoundrels have a mysteriously high probability of having a heart of gold, for instance. Rebellious princesses are also almost guaranteed to join you at some point or another. Even professions which you would normally never take into a battle (let's say Engineer or Dancers) are more than likely to join you by virtue of them being the only ones in the world (or only ones with names).

There are exceptions, of course. As later Final Fantasies have proven, all those people in the unemployment office complaining they can't switch professions are liars who are too lazy to allocate their AP/JP properlly; any cross-dressing pirate can switch nearly instantaneously from being a samurai to being a dancer or chemist. In later games, your job didn't quite matter, you could be a one-armed coal-miner and still have the practical skills of an Ancient?. Also, the use of job to determine likelihood of a character joining your party becomes completely moot in the Suikoden series of games wherein pretty much anyone who has any kind of job whatsoever will join you, no questions asked.

You will never find a main character in a RPG who happens to be a ninja, a magician, or bard. Unless the main character is female, in which she will always be limited to being a magician or summoner.