I guess you can never release dev tools to the public, then, else people would port all the cars and tracks into all the versions, and/or just make a million new ones, which would eliminate the appeal of upgrading.
I think S3 should have something more substantial than just a few new cars and tracks to set it apart. Perhaps weather effects and night driving could be limited to S3 only.
Because I'd really like to have some kind of SDK available, so people can finally make their own content. Unless of course the tools and mods are only made to work with S3. Though someone will hack that no doubt.