I really don't know how often the actual book was updated. I was very surprised to find out that the entries were not written into the book as they occurred. Kind of reminds me of the principal at a school my wife once taught at, who had the teachers send the grades to the office in pencil until he had a chance to look at them. Another thing that surprised me was when I was at National Archives and saw some of the actual logbooks. They have apparently all been rebound, as the writing in some of the books is actually down in the center crack, and could not possibly have been written in the book as is.
But you are right, of course. As a rule, a log book entry in more reliable than anything else you are liable to find. You'll find occasional errors, due to transcription or inattention, but since the originals were usually written as they happened, it's as good as it gets. A lot better than memoirs written forty years after the fact, that's for sure.