Obviously, but the point is that it's the professional part that makes the difference in the quality of a work of fiction. Or anything else. Who would you rather have repairing your car -- someone who just really likes cars, or someone who has years of training as a professional mechanic? Certainly a lot of people in the latter category will also be in the former, but of the two, which one would have more impact on the quality of your car repairs?
I've been a Trek fan since I was five years old. But I still do my research about any Trek project. I rewatch the relevant episodes, I check Memory Alpha and other sources as needed -- I do the same things I'd do if I hadn't been previously familiar. Because just being fond of something isn't enough to live up to professional standards. If you're a pro, you treat it like work, not a hobby. It's a whole other type of engagement with the material.
Not at all. I'm sure most adult fans are professionals at something, which is why I'm stressing professionalism -- because I assume that's something everyone with their own career can understand and relate to. The point is simply that being a professional has a greater impact on the work than being a fan does, as with the car repair example above.
Maybe, but not inevitably. Remember what Greg said when asked if he was a fan of something -- "No, but I can be." Just because you weren't a fan of something beforehand, you can still learn to be one when you immerse yourself in it as research for a project. As I said, I'm more of a fan of Enterprise now, after revisiting it as preparation for my ENT novels, than I was on the day I was hired to do the first of those novels.
After all, fandom isn't something you're born with. Sure, many of us are nearly lifelong fans of various things, but there are also people who've only just recently discovered a show on a binge-watch and become fans in the past few weeks, but are still just as passionate as those who have been fans for years. For me, it was like that with Leverage. I got curious about it for the first time shortly before its fifth(?)-season premiere, and I binge-watched the first four seasons and was quickly hooked. If anything, immersing yourself in a binge like that can make your appreciation even more intense.
So why in the world would you assume that a professional learning about a project for the first time can't become a genuine fan in the process? That just makes no sense.
I don't think you can do this job without loving it. Even if a filmmaker doesn't have a pre-existing love for the specific series, they can still love the process of moviemaking, of making stories work and characters sing. So they'll try to make the best movie they can regardless.
"Extras must demonstrate a working knowledge of the Star Trek universe." 
