I applaud the debate about why so few writers direct, although as an agent my experience is that most of those who do so fail to see that if they are any good as writers they should want a better director than themselves, and if they think they are any good as a director they should want to direct better scripts than they can write. Too many writers are forced to try directing because they can't sell their script to anyone and because the industry appears to love writer-directors.
Instead, I think that it makes more sense for writers to become "filmmakers" by producing. Directing is very skilled, ideally requiring both training and considerable practice doing shorts and television before risking a feature film. The September issue of ScriptWriter Magazine has an entire editorial about this, because it will be a more successful way forward in my opinion as the technology becomes simpler and less expensive. There is also an article on how films are sold and distributed, essential knowledge that few writers have.
We are entering an era of UKP 50,000 to UKP 250,000 feature films, some of which will make a great deal of money even if they do not get theatrical release. It will also be an era in which theatrical release becomes less important.
Remember, Julian's idea of a writer-producer dramatically shifts the balance of power. Now the writer-producer can fire the director!