Hi Antonella;
In my opinion and experience, cocoa powder is the bottom of the barrel in the industry, and I would never add it to chocolate that I make.
Yes, you are right that big manufacturers do add it to some of their products, but it's never a decision predicated on creating a quality product. It's a decision driven by profit.
In the commercial candy bar industry, the margins on chocolate bars are very very slim. Given that (aside from vanilla) cocoa butter is the most expensive ingredient in a chocolate, and cocoa beans are the second most expensive, it comes down to mathematics - minimizing the expensive ingredients, while maximizing the intensity and flavour.
The problem is that cocoa powder is very astringent and often muddy tasting, and if anything detracts from the flavour of the end product. I have never seen a case where it contributes positively to the end flavour of a good quality chocolate.
That's my experience.
Brad
updated by @brad-churchill: 10/14/16 05:07:10AM