My two cents:
I'm not smart. I love geocaching, I love looking for all kinds of paper to write my name on. Micros, hiking caches, LPCs, puzzles, whatever. But often, hiders are smarter than me. This seems particularly true of some of the COs in my area. There have been numerous times that I have looked for something many times and not found it, only to find it later or have to be told where it is and I think, "how did I not find that before?"
So knowing that a DNF will probably cause others not to look for it, I am hesitant to log an DNF because of my own stupidity.
Not that I'm against logging DNFs. Having since then become a CO myself for a few caches, it's nice to know that someone took the time to go to your cache no matter what the outcome. And I would run out the next day and check on the cache if someone logged a DNF. In fact, I try to go out and check on my caches whenever we get rain, just to make sure everything is fine (hey, I live in LA, we don't get a whole lot of it).
If I go to a cache more than once (or more, depending on level of difficulty) then I usually log a DNF. Often, I will email the cache owner directly if I think there may be a problem with the cache or to get a nudge in the right direction. I feel out the situation on a cache by cache basis. But I'm mostly in the "Write Note" category unless I've poked my nose in every single place I could think of for that cache to be hiding on more than one occasion. And that may also come from doing mostly urban caches because I live in the big city. If I was mostly looking for ammo cans in the forest, my philosophy might be totally different.
I will say though, reading this thread has made me think, and that's the best thing of all. Thanks all, for the new perspective.
