At one time I had a diagram of our database schema. Every table with its columns and data types was in its own now with arrows indicating foreign key relationships. Each table was color coded and then then grouped into related areas of concern (user tables over here, retailer tables over there, etc.) We had our print shop print it out at the largest size they could (about 3x5 feet) and hung it on the wall. We referred to it all the time, but it was a pain to maintain as the schema changed and overtime it became too much of a hassle so we stopped updating it. I think the same thing would happen with a system diagram. It sounds good and is helpful, but over time it won’t be maintained and will be abandoned.