I've already done a Tower of Despair run with a Thief (pics or it didn't happen), so I already know a little about said class performs in, and while the Thief will absolutely be annoying to fight with (hungry snack eater, and will eat before the Cleric will be able to heal), they do also have pros.
Sneak Attack is a good way not to get targeted by the Light Sage/Juvenile Snurps, while also dealing damage (and before anyone mentions the multiplier it has, remember it's an attack that can be assisted by Pincer).
Pilfer is literally free snacks to make it up for the appetite, and free snacks for the whole party as well if everyone knows Charity/Share.
Whirlwind Blades/Phantom Strike/Rock Rain are very powerful attacks if there's only one enemy standing to take the hit (my Thief dealed around 70 (x 5) damage to Boss Snurp with Rock Rain, which is in total around 350HP, in comparison my Warrior dealed around 385HP to the same boss with Darkeye Slash/Super Spin Slash. For not being an offense class, I'd say that's not that bad), but once there are more enemies, yeah, those attacks lose their charm. Backflip is useful though, maybe too useful.
And, personally, I found Huge Trap to be useful as well at the Boss Snurp battle. Thief got grilled to death at the Light Sage & Light Lord battle though.
EDIT: I learned how to space paragraphs, wooohooo