I know a lot of people think of it and cringe, but to be honest it's a great exercise in characterization. You're playing with already established characters and trying to build a plot around them, rather then stretching characters to fit the plot.
It's really helped me with plot structure and stretched my creativity in regards to solving plot holes. Plus it's a lot of fun!
If you ever wanna try a writing exercise that helps you practice keeping your characters in character, pick a couple of your favorite characters from your media of choice and have at it!