I'd actually advocate the opposite. III's job system is a far cry from V's flexibility. The less you switch jobs around, the more specialized your character is, and the weaker the chance it'll turn out totally lame in all regards.

Also, a surprising number of jobs is completely useless anyway.

As soon as you can, ditch the black mage for a geomancer. Forget the black mage, magus, summoner, sage, of any other offensive magic class. Geomancer deals consistently high damage completely for free.