"Engine failures are often a very good reason for bailing out, if you are over water, over mountainous terrain or over solid undercast."
Not my cup of tea for aeros, and I doubt many GA pilots wear parachutes when not doing aeros. Mind you sometimes I have thought it would provide an option of last resort in these circumstances!
"But there have been several cases of RAF trainees and instructors bailing out of the likes of Chipmunks and Bulldogs in unrecoverable spins."
Fair point, irrecoverable spins are a good case in point.
"Just read any WW2 history. Getting out of a Spitfire, P-51, or spinning wingless B-17 or a Lancaster on fire in a searchlight cone at night has to be harder than jettisoning the canopy on a slow lightplane, releasing the harness and making sure you and the aeroplane go in different directions."
Yes but height and probably aircraft attitude are on your side.
How many pilots do wear a parachute?