It's not a matter of "belief" or being a "purist" . It's a matter of knowledge, just plain ole common sense and practical >facts< of how things >actually< work. Beliefs are subjective and people can believe anything they want and more often than not beliefs are tainted by the individuals wanting to do something regarless of  any pertinent facts or empirical evidence.

DE while apparently being an inert substance there are a few things that it will do that can affect how the MC works. One is that main workings of the MC is to give the digestive system a rest so the energy otherwise used for digestion is used by the system to cleanse and heal. DE being a solid will activate the peristaltic action in the GI tract  to push it out and 2 being a non soluble fine powder and not having other solid matter to help push it out it can possibly cause a blockage and 3 since it also acts as a vermifuge to kill parasites  this can cause the release of toxins from die off which could make it  very unpleasant and if there is more die off than the system can handle, it can cause a herxheimer reaction and make one very nauseous and sick.