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.