I keep them all except for the obvious miss (focus, really bad exposure, etc.)
I have many images that do not make it as picks, but I do not delete them. And there have been times when that has worked out very well...
One time I did a family portrait shoot. I take several of the same pose. There were quite a few shots where they were obvious misses where most of the people in the shot were not looking at the camera, etc. I had one great shot that was going to be the big sell for a 16x20 print... but the dad had a funny look on his face. So I go back to the 'missed' shots and in one of them where everyone else was looking bad, the dad had a great look.... so I was able to salvage the shot using the dad from this one 'reject'.
Also, on two other occasions I have retrieved shots from the 'reject' pile to provide prints for funerals as they were the last shots taken of the person. Both of these were unremarkable shots, nothing outstanding, but suddenly became very valuable in memories to the families.
You never know... and, yes, disc space is very cheap.