We feel this way for a number of reasons:

We feel that this is the only way to show proper respect to the author of the play. The author wrote the play the way it was written for a reason. It is the job of the director and actors to discover that reason, or at the very least a reason, for what is on the page. To cut material from a play is not only disrespectful of the author's work, but is also presumptuous; who are we to cut Shakespeare? Or Shaw?

Living playwrights are protected from this sort of violation of their work by copyright law, but the poor deceased playwrights whose works are in the public domain have no such recourse. Is not Marlowe deserving of the same consideration as Mamet?

We also feel that this is the only way to show proper respect to our audiences. If we state that we are performing Shakespeare's King Lear, the audience has a right to expect to see the play that Shakespeare wrote, not the play that a director, producer or dramaturge thinks he ought to have written. We also feel that, even if the material is difficult, the creative artists involved in the production should be able communicate that material to the audience, and to fail to make the effort is to insult both audience and playwright.