Thieves are good for low level games... but other than that you're better off subbing them with an RM or Fighter.

Their running ability is good if you're underlevelled for dungeons, but once you get to level 10 or so... any 4-person party can run from anything without difficulty. The higher you level, the easier it is to run. Basically that's why thieves shine in low level games.

Before class change their defense is bad and their offense is mediocre. After class change their defense gets better but their offense is still mediocre. Their unique weapon (katana) is only a sliver better than the Defense. And ninja's natural stats (without a weapon equipped) are slightly lower than a RW's natural attack. So a Ninja with the Katana will have the same physical strength as a RW with the Defense or Sun Sword.

Ninja can learn FAST... but so can a RW... which is why I see it like this:

- for low level games... you'll probably want a thief (probably only one at that).
- if you're not low levelling.. and you already have someone who can cast FAST in the party, you're better off with another fighter. Fighters just have superior defense, offense, and HP.
- if you're not low levelling and you don't have anyone else who can cast FAST, you're better off with a Red Mage/Wizard. Red Wizards are superior in terms of defense and offense early game as well as having magic to back them up. Late game they aren't as defensive as a Ninja, but they compensate with their magic and their physical attack is just as strong (or stronger) than a Ninja's.