Hi Jerome:
that's a fair question. I don't have a simple answer, but let's give it a shot.
My first exposure to this concept came from a textile conservator who called it a "pressure mount". She said she used it often, and recommended it for a tattered, deteriorated antique sampler a customer had brought in for framing. She was very helpful to me in getting that job done. The rest of the direct contact overlay concept was my idea, based on what I've learned about preservation framing, but I've discussed it with conservators on occasion. So, I would say that DCO mounting can be appropriate at least for some precious items, but not for all.
Chapter One of the new DCO book is titled "Caveats and Cautions", and I put it first because it's most important. Is DCO an approved conservation method? I've never seen any attachment method universally approved as a "conservation method". Rather, there are always limitations and exceptions. In my experience, the methods most often used are based on certain criteria according to applications. For example, here are some typical attributes of conservation mounting:
Minimally invasive
Chemically inert or long-term stable
Reversible with no invasive procedures
Non-adhesive
Of course, very few conservation mounting methods meet all criteria 100%, not even these four. For example, the finest hinging with Japanese paper and freshly cooked starch paste results in some permanent change to a document so hinged. In my opinion, it is most important to make informed choices.
I'm sure that conservators, like framers, have differing opinions on techniques. If you have a precious document or other possible candidate for DCO mounting, but you're not sure it would be appropriate, then I suggest you consult your favorite conservator, which is what I usually do.
Now that I've answered your question, please tell us what are the aforementioned thoughts you have on the subject?