We've all heard the basic theatrical truisms: Write from your own experience. Actors and directors should ignore the stage directions. Writers should never direct their own plays.
I won't argue here about the first two (though I can't resist saying that if Shakespeare had felt constrained by the first, he wouldn't have written most of his plays as he wasn't alive when any of them were set and he never visited most of the countries where they take place).
Regarding the third: "never" is a pretty strong word. I know the argument. Writers don't have sufficient objectivity about their own stuff to be able to see clearly where revisions are necessary. They are so in love with their own words that they can't bear to part with any of them. Is this true of some writers? Yes. Is this true of all writers? Nah.
I wonder why it is that this principle is constantly invoked when discussing writers for the stage, but most don't turn a hair at the idea of filmmakers directing from their own scripts. Woody Allen, Ingmar Bergman, Preston Sturges, Billy Wilder, John Huston, Paul Mazursky, Elaine May, John Cassavetes, Nora Ephron, Federico Fellini, Francois Truffaut -- not to mention Charlie Chaplin and Orson Welles -- all managed to turn out some fine movies from their own screenplays, and some even managed to also star in some of their own productions. If anything, directing a movie is usually a longer and more demanding job than directing for the stage, and it almost always requires exercising authority over a larger number of people.
Some playwrights have succeeded nicely directing their own work. My favorite production of Who's Afraid of Virginia Woolf? was the one Edward Albee directed with Ben Gazzara and Colleen Dewhurst. Mary Zimmerman won the Tony award as best director staging her adaptation of Ovid's Metamorphosis. Frank Galati won the Tony for best play and best director for his adaptation of John Steinbeck's The Grapes of Wrath. (And I will modestly add that the production of my With and Without that I directed was received well, too.)
The truth is some people are generalists and some are specialists. Orson Welles, of course, was a legendary generalist. He wasn't shy about his conviction that he could do just about anything as well or better than anyone else. Harold Pinter, too, was a generalist. Aside from writing plays, he acted both in his own work and that of others, and he directed successful productions of not only his own plays but those of David Mamet, Simon Gray and others.
Yes, there are specialists who are at a loss when they leave home territory. I once worked with an Academy Award-winning actor who attempted to direct one of my pieces. He had no sense of shape. He was so intent on mining the truth and emotion of every moment that there was no modulation, and any sense of tempo went out the window. The production was 15 minutes longer than it had any reason to be. He was a brilliant actor, and that's what he should have stuck to doing.
Also it mustn't be forgotten that some people write in rehearsal as they direct. Their writing process is to direct. This is the working method of Britain's Mike Leigh, when he creates his plays and films, and this was how the legendary Paul Sills built his groundbreaking Broadway production of Story Theatre.
Some people are directors and some people aren't. Writers who are also directors may indeed face the problem of objectivity as they stage their own stuff, but many others have the discipline and professionalism to know how to adjust for this. That's what you have other collaborators for – the actors, the designers, the producer, and the rest of the people in the room who are presumably there because they know something about how to make theatre. Directors with any sense will pay attention to and solicit advice from colleagues.
The challenge for the producer is to somehow discern who can wear multiple caps and who can't. But, regarding this issue anyway, the word "never" should be tossed.