There are interesting parallels between the Media Molecule office and LittleBigPlanet -- both have co-op and versus modes, both have dark sides, and both are totally awesome. They feel the game could have existed and risen in popularity even without user-generated content. They want to enforce that concept: they pitched the gameplay mechanics and tactical elements to Sony. The user-generated idea wasn't the focus. It's a fun game where building stuff yourself is merely an option and not a necessity.
Still, they hope what users create is as professional or better than what they ship with the game. Those kinds of talents are in every game with user-generated content, after all. In fact, if people find ways to exploits the creation tools Media Molecule provides in interesting ways, they'll promote it. On the flipside, offensive or poor content will get rated down, similar to our own website's comment system where the "rubbish" simply fades away, while the good remains. We look forward to hearing more as the game draws closer to release.