Don't think that Earth wasn't in danger. Khan had himself a weapon of mass destruction that could destroy an entire planet. No world in the Federation was safe, not even Earth. According to an interview at startrek.com re-posted at our own TrekToday, they didn't, but producer Damon Lindelof did. From TrekToday: “[Damon Lindelof] argued for Khan from the beginning and I argued against it,” said Orci. http://www.trektoday.com/content/2013/05/orci-why-this-certain-villain/ Orci and Kurtzman compromised with Lindelof; they initially wrote the story without Khan and then found a way to insert him in later.