I spoke today with another local contractor that builds a lot of fireplaces. He is familar with where I am working and I asked about the additional depth. His opinion was based on the particular town the work is being done in due more to soil conditions and stabilization in softer soils. As the particular area I'm working in everyone refers to as a "rock" with the clay and shell rock.....he had no concerns after I explained how everything was constructed and how I am finishing it. My thoughts are very similar to his and maybe we are both wrong from a book's perspective but everyone I've spoken with is many years ahead of me in experience and do not express any concern.
While I do understand the point made......my opinion and those I have asked bring it down more to where we are working. Softer soils, hills etc.....different story from this application. I'm sure some will still disagree but I've pulled my info from some of the areas most well respected and if they had bad work, they wouldn't be in business as long as they have. Who knows, out of an itch in the back of my head I may do it anyway on my next one in this particular area as I do not want a failure in my work.