I had a friend at college who loved Seven and Fight Club, and he said he "loved Brad Bitt movies" cause those 2 films were so good and bought all the Pitt movies he could. It never occured to him to look for more by the same director or writers. Another friend of mine bought all the films with Kevin Spacey he could find cause he loved American Beauty so much. But nothing else by the same writer or director of A.B.
I'm not saying that people shouldn't like a star and follow a star, but both times it seemed like it was the story of the movie they loved, yet all they could think of was to look for the actor.
On the other hand though, if you trust a star to choose good roles and know he only makes quality (Like Spacey in LA confidental) then you know your going to get a good movie, for example Al Pacino only seems to appear in good thrillers. So maybe its not a bad method after all.