Paul Graham argues in an insightful post on the essential difference between a maker's schedule and a manager's schedule that the two don't mix. "The manager's schedule is for bosses. It's embodied in the traditional appointment book, with each day cut into one hour intervals. You can block off several hours for a single task if you need to, but by default you change what you're doing every hour." On the other hand, "people who make things, like programmers and writers ... generally prefer to use time in units of half a day at least. You can't write or program well in units of an hour. That's barely enough time to get started."
Seems true to me, even though I also think it is better to try and get started on things when one has an hour or two than not even to try. There is always note-taking or research to do, in any case.
It does seem to be true, however, that GTD is an approach well-suited to those who manage, not to those who create. Since we all also need to manage, we may be misled into thinking it is well-designed for the most important parts of our lives as well. But it isn't