It is my solid opinion that fan fiction is in fact good for beginning writers. Why? Because the essence of it comes from someone else’s mind. It comes to a writer through a movie or a book, fully formed and preferably done by someone with experience. It is easy to get immersed into a well-formed universe and become a part of it. Fan fiction can come easier than original fiction precisely because it already has a visual sense, whereas your own setting/plot is pure abstract; it only exists in your mind, therefore it is not fully formed until you put it on paper.
Fan fiction is a great way to practice. Some people stick with it forever and don’t write original fiction at all. Most of us, though, start to crave to invest our effort and time into something that we can call our own. That is where original fiction comes from.
Play to learn. Learn to master. Master to succeed.
Martin Vavpotic's Blog 
“Most of us, though, start to crave to invest our effort and time into something that we can call our own.”
I was casually agreeing with your post until I read this line, and then I was ardently agreeing with your post. That is exactly what happened to me. One day, I just suddenly wondered why anyone would write fanfiction when they can never claim the characters as their own. Even though they’re putting so much effort into creating stories for these characters, they will never be able to publish their stories! That day, I stopped wanting to write fanfiction.
Fanfiction was definitely my starting point. I read over my stories once years later, and I instantly regretted ever releasing them to the world wide web. They were so terrible. I apparently thought that ellipses could end every paragraph to convey deep-seated emotion. Thank goodness I stopped that nasty habit.