Beyond that, I think the biggest disconnect we're having here is how much Thieves' Cant defines your character. Nowhere in the description of Thieves' Cant does it say you're actually a thief, only that you picked up their language. You could easily be a freedom-fighter who uses dirty tactics and Thieves' Cant to communicate with other rebels. Or you could be a spy who learned Thieves' Cant so that he can easily contact informants or thieves when the need arises (and it often will!). Or maybe your character is part of some other sect of cunning, sneaky-types who use a secret language to communicate.
My question is, how is Thieves' Cant more restrictive than a wizard's spellbook and method of preparation? Or a warlocks pacts? Or a sorcerer's bloodline? Or a Fighter's combat training? Or a Cleric's domain?