Ask yourself, are you going to put in the same amount of design and content work into the mobile site as the main site? Really? Aren't you just changing the links to buttons, and reusing the content? Be honest.
The one exception is if you are implementing a proper full stack html 5 application. With truly valuable features over the standard website (but then why haven't you done that for the main site too?). Even then, always have a path back to the main site, and keep an eye out for how often that link gets used.