From the owner's point of view (at least me as an owner), I enjoy reading the online log entries, especially when the cacher has made an attempt to tell me how much he/she enjoyed finding the cache. This is part of the fun of setting out the cache.
As a "finder", I always log. My entry at the cache may be very brief, especially if it is a cache in an observable area, but I try to make up for the terse entry with a good online post that will tell the owner how much I enjoyed the cache--at least for the ones where it is obvious that the person placing the cache made a real effort to make it an enjoyable experience.
I suppose if someone chooses not to log, that is their business, but I wonder how many people would still place caches if no one provided feedback and encouragement?