Just think about it for a second. It's getting assets vs. saving a quantifiable number of turns. Chad's role is less unique after Astohl joins and not unique at all for all intents and purposes after Cath joins (let's not forget the existence of Chest Keys either). Lalum/Elphin's roles are completely unique as long as they exist.
And by the time you get near endgame, it's not worth deploying thieves for their utility because they're such severe combat liabilities, whereas a dancer by nature reduces his/her own combat liability by helping eliminate threats in advance.
And by the time you get near endgame, it's not worth deploying thieves for their utility because they're such severe combat liabilities, whereas a dancer by nature reduces his/her own combat liability by helping eliminate threats in advance.