The Thief can be very useful at times. It has the highest speed, so it usually attacks first in battle. You will want to use it's steal command often, especially on the bosses because they usually hold rare and/or powerful weapons and items. Besides the obvious, the Thief is automatically enabled with the Secret AND Dash abilities. The Dash can come in handy if your trying to get all the treasure in Karnak Castle or if you want to get to Gogo really quickly. The secret is also good if your strolling threw a town (esp. in Mirage Village).
Lancer >>>