As a cache owner, I like to know if someone is looking for my cache or not. I have one cache that I placed back in November that has only been found 8 times and it is just off a main road, and for the most part, a pretty short walk. It is a 2 stage multi and I hope a little tricky. There has not been a single DNF logged, so for a while I was wondering if people just weren't interested in finding it or if they weren't logging DNFs. After almost 2 months of being there, and only having 3 finds, someone posted another find at the end of January, saying they can't believe they missed it last time they were there.
Well, at least I now know that someone was looking.
Since then, I have made a point to try to log all my DNFs, no matter how hard or long I look. It lets the cache owner know that at least someone is still interested in the cache. Especially if it hasn't had a find in a while. I'll even posted a note recently saying I didn't even look for a cache because the road to get to the cache was partially flooded. The cache was still accessable, it just would have involved wading and walking a ways.
The other nice thing about logging DNFs is that you can then go back and look at which ones you couldn't find easier to see if anyone else has found it since, rather than trying to remember which ones you need to avenge.