Alfalfa is another such example of crops that actually produce N and restore SOM, and there are many. I have no reason to doubt that everything you said there is true. I guess the same might be true of cannabis root. You can leave the root in your soil and it will decompose and give back whatever it contains. It certainly does no harm....One example is the use of legumes as a cover crop. Farmers will plant legumes to restore SOM (soil organic matter) and improve the N (nitrogen) that are present and collected on the legumes roots. Most of the N is absorbed from the plants leaves from the air and stored in root nodules. When the legumes die back or are harvested, those roots give N back to the soil in the decomposition process.
I guess the only way to really know, as fact, if cannabis root is just bursting with nutrition is to have it tested. There isn't much I could find on the web to corroborate what, if any, nutritional value cannabis root in fact has.
Your analogy to legumes etc is nice -- yes, some plants' roots give N back -- but it doesn't establish as fact what was asserted by nunyabiz, namely, that cannabis root is just full of nutrients. I don't think it is honest to expect others to accept this kind of assertion without questioning the source. That's all I wanted to say.