She hasnt had any lead roles, she has had supporting roles.
Once again, you still dont get the point, people do not go to see a movie purely based on her being in it, unlike John Wayne, Sean Connery and Johnny Depp etc, because she doesnt have the kind of screen prescence they have/had.
Your second example fails because people like those actors, and go to see them for a reason, either they can act or people find them funny.
Like or hate them, those three have a fanbase for a reason.
Regardless of whether they have been in shit movies or not is neither here nor there, the fact is, with people like Depp, shit movies have been made watchable just by putting him in it.
Are you trying to say that its wrong to like an actor so much that you will give any film a try just because of them?
Lets face it, how do you know a film is shit until you see it?
Do you only believe what others tell you?
A lot of people here say that the Thomas Jane Punisher film was shit, but you like it, does that mean you shouldnt like it then?
Depp, Sandler and Willis have all earned their fanbase, so why the hell should that fanbase not feel that theres a good chance that anything they are in is gonna be good?
Do you also say that someone shouldnt buy all albums by a band they like, cause sometimes bands produce clunkers of albums?
A good actor/actress/comedian will always garner a loyal fanbase, and will always attract people who have enjoyed their previous films, but someone the likes of Megan Fox will never be enough to sell a film to people.
As a good example of one of her upcoming movies, Jonah Hex.
People that dont generally like westerns, will not go to see it.
Now if Depp or Willis was in it, you can bet that the clout that they bring to it would certainly attract their fans, even if they do not normally like westerns.
Its the same reason why people who were John Wayne fans watched Waynes war movies, when maybe normally they were not a fan, but were fans of westerns.