Thieves are valuable for their ability to flee... but unfortunately that's about all they're good for. They're outclassed (heh) by everyone else.
Fighter/Knight - Too expensive and equipment dependant
Thief/Ninja - Not worth being handicapped half the game for a subpar Red Mage later
Monk/Master - No customization at all, pretty boring
Black Mage/Wizard - Offensive Magic sucks and I hate status magic
White Mage/Wizard - Items will do most of their work anyway and I hate dealing with limited MP
Red Mage - Has all the strengths and weakness of the BM, WM, and Thief rolled into one
Thieves are valuable for their ability to flee... but unfortunately that's about all they're good for. They're outclassed (heh) by everyone else.