GTD is supposed to be a pick and choose the parts that will make the most difference in your life and adjust to what works for you. If your not "a manager with a wive and kids and maybe a foundation to run on the side." then you don't need the full, but there are often useful things in there that are useful.
GTD is not religion (or at least it should not be - with some people it is), you won't go to hell or something if you don't do everything perfect.
I'm not a manager, but have a wife and kids and used to run a foundation on the side. The overhead of GTD only got more painful, while my ability to sustain focus diminished, so the whole thing broke down.
If you don't usually forget tasks/events or freeze up because of analysis paralysis, you can get by just fine without it.
GTD definitely brings an overhead that doesn't pay off in a reasonably simple life