I think you should always write something.
Every flight is memorable for some reason and when I look back at all of the empty notes I wonder what I did with my time in the air.I always make a point of flying with someone so their name always goes in (they may be famous some day). I now resolve to fill in the notes on each flight.
Remember also you are not limited to the one inch of notes , you can write on the next line, and the next one and heck log books are cheap.
Imagine picking up the logbook of a pilot and reading 'Took the kite over the Solent, watched the sun setting over Poole and tailchased a tiger back to base. Cricket team were playing as I landed' versus 'Local flight'. When someone says to you , how did you spend your £10,000 which logbook would you rather have?