I am sure there is no legal need to log a flight in your personal logbook. If you forget to log a flight, that is not a crime.
Lots of "old" pilots can't be bothered to log flights they do.
The incentive to log a flight comes from the need to build certain airborne experience towards certain licenses/ratings. If you are not wishing to (ever) acquire any new licenses/ratings, then there is no need to log anything beyond the minimum required to renew the PPL or whatever.
It is logging flights which you have not done that is dodgy.
Similarly, not logging flights in the maintenance records is dodgy.
FAA don't need you to log your personal flying hours after your training... imagine that!
I think that one comes from the FAA PPL not having a JAA-like min-hours requirement - there is only the BFR which is a flight with an FAA instructor.
You can do the same under JAA. If you have no hours logged over the previous 2 years, you have to fly with a PPL examiner. The end result is the same: no need to log any hours.