Sergio,
You are, of course, quite right about the perils of drawing from a photograph. I wish I could convince even just a few people to sit long enough for me to draw a complete portrait from life. I had to bribe my son to pose for this photo (he only allowed me to fire off three shots---good thing one of them was fairly usable) before he bolted for the house!
At this stage in my development, I am not a "quick draw". I would need for my model to endure several protracted sittings in order to achieve a satisfactory drawing from life. Every time I attempt to draw a portrait from a live sitter, I'm intensely aware of how bored and uncomfortable they seem to be getting (after only a few minutes, even!), and it makes me feel quite anxious. I also worry about their reaction to my efforts after they have endured sitting still for so long. "A model! A model! My kingdom for a model!"
I like your analogy of the reference photo as, "ready to heat pizza". It is quite true for all but the most accomplished artists.