I would say the existence of Thieves' Cant either indicates that WotC intends to create a massive number of highly specialized classes, or that they made a design mistake. From what I have read, the developers do not intend to release a massive number of highly specialized classes. No other classes in the material we have seen have been highly specialized with regard to character background - there are mechanical specializations, as you have noted, but there is nothing that forces a Fighter to be a soldier. There is something that forces the Rogue to be a very specific type of criminal.
Spoiler""The goal at the moment is to include all the classes that were in the first PH style book for each edition." Specifically mentioned by WotC staff so far are: fighter, cleric, wizard, warlock, sorcerer, bard, paladin, psion, barbarian, monk, druid, warlord, assassin, rogue.", source
The Rogue in 3.5 was much broader in what characters it could represent, because it wasn't mechanically tied to a stereotype, the way the D&D Next Rogue is with Thieves' Cant. This significantly broader class existed alongside other similarly broad classes. The only base class that had a constraint on character background was the Druid (with Druidic as a specific language known to every Druid), which isn't well thought out either.
I think that backgrounds should be used to represent a character's background. In this case, my character's background is determined by the class I've selected, rather than the background I'd like to select. Why do you think that a class should determine a very significant part of a character's history, rather than allowing a player to choose their own background?
We're actually in agreement for the most part. We both agree that Thieves' Cant is acceptable if there are enough classes around that many characters are feasible. I simply don't believe that WotC will release enough classes to accomplish this (as it seems they're generally relying on backgrounds for this sort of thing in D&D Next), while you do. I believe that placing Thieves' Cant into a "Criminal" background that anyone can select, regardless of class (surely there are criminals out there in other classes who can read a gang sign) is a more appropriate fit with the design philosophy of D&D Next.