I think Thieves' Cant is in there on purpose as a test. Like when everyone wondered where the Cleric's Turn Undead was, they want to see what playtesters consider necessary to D&D, even if it isn't used much in actual play.

That said, I don't like it. A class should be what your character does, not what they are. If you make a bodyguard/enforcer for the criminal underground Fighter may be a class that's the best fit for your idea, but they would be much more likely to know the Cant than a Gentleman Thief who took to it as an escape from the ennui of High Society.