"Among thieves, there is a secret language, a way of communicating between members of the criminal underworld that rogues know and use. Creatures hearing you converse in Thievesí Cant might think you say one thing when you are actually saying something else entirely.
Benefit:You have learned the secret language of thieves."
Name me a setting that actually has a "secret language of thieves"? Because off the top of my head, established settings like FR, Eberron, Golarion, Dark Sun, etc. don't have a secret language of thieves.
I think they learned it the way most real life criminals learn it - by experience, rather than teaching. As an example, check out people like Bill Mason, and his autobiographical book Confessions of a Master Jewel Thief - and note that Bill doesn't speak in a magical, sooper sekret language that only jewel thieves can understand. Nor do I think that every Rogue needs to be a criminal; a military specialist who's had formal training within armed forces on sabotage would have no reason to speak in this secret, magical, criminal language - but he magically learns how to do so the second he receives his military training. I should be allowed to choose my character's background - and unless they intend that every Rogue be familiar with crime, they should change it.Yes, which could happen in any number of ways. A gentlemanly thief doesn't just jump out of bed one day and steal the crown jewels. He needs to spend a long time learning how to open locks, sneak around, disarm traps, bluff guards, and fight dirty. Where do you think he learned all of this, a book?
I don't have any problem with giving the thieves' cant to a background, I simply don't see why it can't be on the rogue, as well.
Unless the designers are intentionally restricting the range of characters that can be expressed by the Rogue (to characters who are intimately familiar with criminals, in settings where thieves have a secret language), Thieves' Cant should only be part of a background that is available for any class (because every class could have a criminal background).