Yup, I log my dives for most of the reasons already posted:
1) Gear configurations-sure helps to fine tune the system
2) Remember info about previous dives to same location. Particularly, I log directions, courses (degree headings), and distances from landmarks so that I can explore new parts of even familiar reefs and sites.
3) I have been known to record what my partners have done...I've even "earmarked" a few to indicate whether I really want to dive with them again or not.
4) I've been asked to produce my log. And I am reasonably certain that if I had not been able to produce it the operator may have let me dive, but my dive plan may have been modified for me until he was confident that I knew what I was doing.
In fact, on several scientific expeditions, I had to produce my log before I could even be considered for the crew.
5) Hey, I'm getting older...it's fun to look back at some dives and have my memory jogged about elements I may have forgotten
I do admit that logging dives isn't a chore for me. It takes possibly five minutes (usually less), and I usually don't log the mundane. It's new or interesting items I record in detail. Remember, this is supposed to be FUN!