Hate to start this way but it is true, "In the olden days..." when I started diving, 1966, no one mentioned a log book. They also didn't use BCs, pressure gages or safe second stages, infact it would have been hard to add to a Royal Aquamaster, bit that is a differentt story for another time.
My first 13 years are not logged and I wish they were. The logs might help jog the memory and organiize the scattered thoughts form those days.
I started logging in 1979 and did it sporadically until I became an instructor in 1982. Since then I have been pretty anal about it. The data in the log helpss me see if and when I am on top of my game and also helps me remember details that may otherwise be forgotten.
Noe I use a downloadable computer and even went so far as to buy a color printer so I can reap full benifit from the program. Whether or not you keep a log is up to you, several operators won't let you dive until they see the log and if you are going to become an instructor or take tech courses you will need the data.
Someone should do a poll about this, 10 or 20% groupings of how many of a diver's dives are logged.
As for the downloadable computers, if something did happen to a diver it can be accurately recreated and studied to help prevent it from happening again or to someone else, and in the worst case seneerio, it is the only witness.
Greg