Whopity's quote suggests that a PIC is acting illegally by
not logging a flight.
Is that true?
Why would one not want to log a flight? There's a good question
You are an instructor, and "something" happened on the flight which you don't want to be responsible for but you
are responsible because it was a G-reg and the other chap was not rated for the type (don't ask me how I know about this one).
You claim to be somewhere else than on the flight, for whatever personal reason.