In actual fact guys you're both right. The lights aren't dimmed categorically for one reason. In an emergency, particularly crash like, situation there is a good chance that the lights will go out. By accustoming your eyes to that that level/lack of light will instantly help you if an emergency/crash/evacuation occurred. It doesn't matter whether there are exit lights or reading lights, the fact that the cabin has dim lights will help. It doesn't really have anything to do with accustoming your eyes outside of the aircraft.
Another reason, as PENKO says, is to assess the conditions outside though the primary hazard is fire which can be easily seen anyway.
In the past there have been various reasons banded around. I remember when I used to fly as a nipper back in the early 90's there were various reasonings actually announced on the PA. One such example was to reduce the effect of night time departures on those living on the approach/takeoff. It was almost like they were trying to shield you from the real reason.
Anyway, it is still one of the most common questions asked by passengers.