Mark,
my appologies for not actually posting a reply which addressed your original question.
have just re-read what you asked and my responses. You are absolutely correct, the lights are dimmed so there is a chance your eyes might be "dark-adapted" when the power fails and you need to exit the aircraft into a dark environment.
However, if the CAA/FAA was really determined to enforce this principle to enhance the safety of the passengers why then does it allow you to continue to read by means of your own nice bright individual reading light???????? Spoiling your own night vision and that of the poor bugger sat next to you???
True flight safety only comes when you lock the planes in the hangar...
Fly safe
T