Kaulen Fuhs said:
At least in my eyes, if the writers for fanfiction were ever truly talented, they'd write their own shit. At least, that's how I see it.
Writing isn't something you're born with. Even the most talented writers have to practice to hone their skills and styles. And even in formal writing courses, a common way to practice writing is to start with a pre-determined premise and build a story on top of that. You're given some sort of setup, like a situation, a set of characters, or a general theme to follow and then you focus on the other aspects that aren't already determined by the premise. This is an effective kind of writing exercise because, depending on the setup, it allows the writer to specifically target certain things they need to practice. For instance, if they need to practice writing exchanges of dialog, then the premise can have something to do with two characters who are arguing about something. Or if they need to practice writing in a certain perspective, having a set plot to follow can allow them to approach the exact same story in two different ways.
Writing with a pre-determined setup is a common exercise in writing, just as studying and copying the works of other artists and the old masters is common for visual artists. There's even a fancy word for making a copy of a work purpose of studying it: pastiche. So the way I see it, fanfiction is no different from those sorts of writing exercises. People utilize just the cast, or sometimes the cast and certain aspects of the plot, and then make their own adjustments from there. They write their own dialog, sometimes add new characters, or if the work was originally written they might even change the perspective from first to third person, or third to first. The perspective in which a story is written has a huge impact on how the plot is revealed, so to change that perspective provides an interesting challenge to the writer, and opens up new possibilities while also restricting others that were previously possible.
If writing fanfiction is what compels a person to put their fingers to the keyboard and work out their own approach to writing and tackle the problems that come with writing stories, then I say more power to them. To say the practice isn't valuable would be like discouraging someone who exercises by playing with nerf guns in a live-action society rather than going to the gym and performing a regimented workout. Just because it's informal and they're having "fun" doesn't mean it is a total waste. Who knows, maybe getting their feet wet in fanfiction will inspire them to move onto greater things. Maybe not all, maybe not even most, but they're having fun and there needs to be a place for diamonds in the rough to begin their journeys.