My friend just answered that it is a combination of the "extra eyes and ears" for the flight crew (remember the 737 engine fire at EMA, the pilots shut down the wrong engine), should something go haywire during taxi or t.o. / landing, and the fact that this way, the difference in light level between inside and outside is minimised. That is also why the lights dim for taxi in the dark. Should the need to evacuate arise, then people don't have to jump into a black hole to slide down the chute.
To the best of his knowledge, this is mandatory now for all airlines.