As an experienced geocacher, I wish you would log your DNFs. If you didn't find it, A the cache isn't there. B It is there and the owner didn't rate it properly or C As a novice, you took on a cache that was beyond your ability.
But above all, a cache owner wants to know whats going on with his cache. The least someone who looks for it can do is to give the owner feedback. Its called courtesy.
or D) I went for a walk and looked for a cache along the way, but ran out of time. Now if I thought anyone would benefit from knowing just how long my lunch hour is, or whether I only had 2 hours between the doctor's appointment and the soccer game, I would have a reason to post it. However, if (as I mentioned)I notify the cache owner if I made a thorough search and came up empty, I think that is sufficient enough to meet your definition of 'courtesy'.
Obviously, this is a touchy subject for some on both sides of the issue, so I'll know to stay out of this one in the future.