Conventional tungsten filaments loose tungsten atoms that stick to the inside of the envelope and eventually darken the lamp. My understanding is that a halogen lamp looses tungsten as well but the envelope is so hot that it doesn't stick and winds up redeposited on the filament extending the brightness and longevity.

 

This replenishing cycle only works well if the envelope is hot enough. As the intensity is reduced, the efficiency is also reduced and the lamp could fail earlier. You would have to do some research to get good numbers, but I think there is a intensity where the the lamp's life is shortened. Running the lamps a little lower probably does increase life expectancy as does running it very low.