Unless it's the back of the lens cap, a shot of my backside or ground, a total failure of exposure/focus, etc., most everything goes into the deep archives just in case. Then I purge 60-80 percent of the images and those are in my JBOD farm under the desk.
Why? First, I remember the story of Dirk Halstead and the story of Monica's blue dress. He archived his work, the rest of his colleagues chimped and dumped their files... Second, when I edit, many times it's initially in the field of under deadline. Having the similars stashed in the deep archives saves me when I choose an image and then find out it's a little off in focus, composition, gesture, etc. It happens a few times of the year, but not too common.
I seldom, if ever purge in the field since the back of the camera is seldom a good way to judge an image and if you're chimping your pix, your missing life unfolding in front of you.
This way, I figure I'm covered. I've got fodder for visual notes, I've got "B" roll pieces to add to my story and I have back-up for my working files.
I also have my history of my vision, what made the cut, what didn't and if I return and shoot again, I can then see what I should concentrate on or improve.
Speaking of history, sometimes a little snap that seemed so inconsequential at the time becomes a cherished piece of history when the world out there changes. How many times have I simply said "I shoot that later when I have time or the conditions look better..." only to come back later and there is no later....
As always, find a system that works for you and keep at it!