Personally, I think there are plenty of rogues out there who have no reason to have that particular ear.
And I can see how it would be annoying to ignore things you're not supposed to understand. It isn't even fair to the persons talking in thieves cant, who aren't realizing they are actually being overheard.
A thief is a thief really, it shouldn't be any different for character approving than any other ear request, even easier I'd say.