Maybe I'm being very cynical today - but what's to stop the mobile companies from excluding those "3rd party" sites from access to their deals? After all, if the game plan is to force consumers into the carriers shops and onto their websites, then this seems like a natural progression.
I know that there's the "it's anti-competitive" angle, but that hasn't stopped some of the insurance companies (e.g. Aviva, DirectLine, etc) from doing exactly that. So if you want their products then you have to approach them directly.