Lots of Jobs in Final Fantasy 5 offer many great, and bad, Abilities.
Some of them are worth getting, and some others are negligible.
Let me give you an example: the Knight.
This Job starts off with some decent Abilities, Cover and !Guard, which are somewhat useful for, say, the Monk.
However, the real great Ability from this Job is no doubt the Double Grip, which increases your attack power by x2.
After that Ability, the Job isn't interesting anymore.
From then it just gives you those annoying EqpX Abilities, and you can easily skip the Knight for a Sorcerer with the Double Grip or something.

This goes for lots of other Jobs, and some of those are just temporarily uninteresting, e.g. the Wizards; they won't reach LV5 White and Black until (SPOILER)the second world and therefore there's no real reason in leveling them any higher in (SPOILER)the first world.
Same goes for the Monk as with the Knight: the last three Abilities are just HP+X%, working upwards,
and are not very useful at all.