Just from personal experience, when I've taken the time to keep a detailed log, whether it's work, weight lifting or skydiving, I find I improve much more quickly. But it's also a drag, so I can totally see why many experienced jumpers don't keep detailed logs.
I guess it depends on where you're going with it - I know some guys who have detailed flyfishing logs, but I just like to fish. Coming Soon: An exciting tag line