I'm not trying to start anything, but I just find it hard to believe that chicks can survive without a heat source. Or let me rephrase--if they do *survive*, I imagine it can't be very healthy or comfortable for them.
My chicks are in a warm room in the house. I've been going through a bit of a hassle with lights--they're either too hot or too cold, and when they're too cold, the chicks huddle under them and get noisy. When they have done this, the temperature has only dropped to about 80 before I adjust it again. (As an aside, I finally did get the heat figured out--just took a few days of troubleshooting).
Point being, if chicks get really cold and unhappy in 80 degee conditions with a light shining on them, I find it impossible to believe they'd be happy or thrive without any light at all and subject to room temperature or worse.
Or in other words, just because I'm sure it is possible to brood them without a light, but certainly not commonplace or ideal for the chickens, why do it? Heat lamps aren't very expensive. And despite what you said about efficiency... lights are efficient and are used by the big factory farms. It isn't a pampered house chicken thing--it's just standard poultry brooding practice, whether you have 1,000 or 2 to brood.