I don't know about everyone else, but I find it annoying to see this on the jacket of a DVD or at the beginning of a movie.
As far as I can tell, the "based on a true story" = fiction, but for some reason, supposed to be more interesting. I challenge anyone to take a "based on a true story" movie, versus one that is just made up and see if they can tell the difference.
It's just a silly distinction that annoys me regularly and plays no part in whether the movie is good or not.
Then, there are a group of people who say, "But it's true!", which it isn't. I guess that's the target audience.