Apparently the original plan was that the jobs would be more role-focused and therefore best for group content, while the basic classes would be more versatile and therefore best for solo content. Some of that plan remains in the fact that classes get more cross-class slots and can choose from a wider range of skills to use in cross-class slots. Jobs on the other hand can only cross-class from two other classes (ACN and THM in the case of White Mage) and have fewer slots for doing so. However, the stat boosts you get with the job generally outweigh that extra bit of versatility, so most people prefer to just stick with their jobs even when they're soloing.