Bit of a late reply to this, but that's definitely an erroneous assumption, haha. Just because it's possible and efficient to level all jobs on one character definitely doesn't mean that everyone prefers to do it. I know at least four members of my FC have the full 8 characters on our accounts, while another is sitting at 6 or 7, and plenty of others have several alts as well. You'd be surprised how many people just prefer to have some variation.