flapsforty, I do not know your airline, I am talking about mine. One of the largest in Europe. There is no reason to keep the lights on at bright. Relax.
Regarding the eyes being accustomed to the darkness outside, well, the brightest lights in the Airbus aircraft are the signs indicating the emergency exits and those come on when the gear are down. ANY night vision you might develop with a darkened cabin will be ruined once you walk past one of these signs near the exits during an evacuation. I go with the explanation that it is easier with a darkened cabin to look outside through the windows to assess the damage before opening a door in the event of a mishap.
Funnily enough, in the cockpit the pilots are not even allowed to switch off the lights at night during takeoff! What night vision?
Final point regarding night vision, if night vision was important to the CAA they would have banned the use of those bright reading lights during landing and takeoff...