For people who want to focus but cannot always, GTD is a floor. While doing deep work, he should do nothing else. When he is not, or more importantly when he cannot, he at least takes care of small things and chips away at his big projects until he's pulled back into them completely.
I like a literary analogy here: I would recommend GTD to an obituaries editor for everything. I would have recommended it to Samuel Coleridge to keep him working during his low periods, since they prevented him from finishing his work. I would never recommend it to a busy genius like Shakespeare or Dosteyevsky for fear of ruining their ability to write (I assume I would have recognized their genius as a contemporary; please work with me.)
I like a literary analogy here: I would recommend GTD to an obituaries editor for everything. I would have recommended it to Samuel Coleridge to keep him working during his low periods, since they prevented him from finishing his work. I would never recommend it to a busy genius like Shakespeare or Dosteyevsky for fear of ruining their ability to write (I assume I would have recognized their genius as a contemporary; please work with me.)