Writing prose does not automatically translate to being able to write effectively for film/TV (and vice versa, for that matter). The demands of the different mediums require different skill sets, not all of which are interchangeable. Same with comics. I'd love to write comics (Trek, for example), but I know I don't possess all the necessary skills to do so.
Since Trek, at its core and when it's doing things right, is about the characters with the futuristic setting and toys as a backdrop for exploring said characters, then having "true" SF writers isn't necessarily a requirement for writing good Trek stories. I think the more effective use of such an individual or individuals would be as story consultants when the plots call for heavy SF elements. After all, Gene Coon wasn't an SF writer; neither were Michael Piller or Ronald D. Moore.
Just my $.02.