Things fell into place when i read it. Robert Fawcett just put words and examples on something i was trying to figure out for a long time.
I love Robert Fawcett and i think this method is very important to study if you want to understand how were made illustrations in the 60s. Bernie Fuchs and Austin Briggs are also good artists to analyse while practicing the light and dark patterns method. Andrew Loomis also talks about it in is excellent Creative Illustration book.
It becomes clear that using a single light source for your scene is necessary if you want to design light and dark patterns. Several light sources would confuse and distort the light and shadow relationship and would work against contrast and legibility. There is an entire chapter dedicated to it in Andrew Loomis' book.
Also, and that's what s great about it, using a single light source allows to remain realistic. In the every day life, it is pretty rare that several principal light sources are competiting. Think of the sun, the sky or the light on the ceiling. Our eyes are used to see single light sources everywhere, so light and dark patterns only make sense in our brains if they are originated from a single source.
I also found interesting to focus on contrasts and design rather that rendering. Not that rendering should be left on the side but there is an order to follow. I used to concentrate on rendering to early in the process, without locking the light and dark patterns.
This is a good exercise and I think I'm going to experiment more in that direction.