Final Fantasy V uses an evolved version of the job system that started in the original Final Fantasy and was refined in Final Fantasy III. It has significant changes over the old systems which I feel are vast improvements. Here's how things changed...
In FFI you could pick a job at the start of the game and then make no other decisions. This meant you had lots of different potential parties you could play with, but couldn't adapt on the fly to interesting challenges. You could evolve all of your characters at once with a sidequest most of the way through the game but there wasn't really a choice there. The upgrades were all strictly superior (well, except for a stupid monk bug) and locked in. A white mage became a white wizard, never a black wizard.
In FFIII you could change your job at any time you weren't in combat. You acquired jobs at different times so often you'd want to change jobs to try new things out or to set up for specific challenges. Unfortunately they built in two mechanics that punished changing jobs. There was an overt punishment where you sucked for a number of fights after changing jobs, and there was a subtle punishment where how good you were at a job was directly related to how many levels you had in the job. Switching around to try different things out was just a bad idea. You also got no benefit for spending time in another job (unless that job had high vitality) after switching out.
In FFV they fixed all of that. Now there's no penalty at all to switching jobs. You might need to juggle a lot of gear sets, I guess, but you don't get a big stat penalty for switching. And there's a benefit to switching jobs! Each job has a blank ability slot which can be filled in with an ability learned by leveling any job. Maybe it's from your current job. Maybe it's from some other job. Want to be a fighter who can heal? Add white magic as your secondary ability. Want to be a white mage who can steal? Level up thief a little, learn the steal skill, then switch to white mage and set steal as your secondary ability. There are tons of different abilities you can learn along the way. As an end game bonus if you've mastered a bunch of jobs you can switch to the bare or mimic job and get the benefits of every mastered job.
The really awesome abilities are all learned after mastering a job, so there's still an incentive to stick with one job. A reason to specialize and a reason to diversify? Sweet!