This is a great question! A Ninja access opening, as the name indicates, is meant for martial arts aspects of endodontic therapy and has very little in common with Real World Endo! 😉
The idea of saving one additional millimeter of dentin while concurrently increasing the chance of missing hidden canals and introducing complex file paths that torque the file is an example of increasing the complexity of the procedure without scientific proof of a significant clinical improvement in case outcome.
As the name indicates, RealWorldEndo is proponent of increasing success rates within the realities of clinical practice. Unless the Ninja access opening folks can produce evidence that their reduction in the convenience form is a clinically significant contributor to improving case outcome, their increase in complexity of the procedure is unsubstantiated and unnecessary.
The way I see it, the burden of proof lies on the shoulder of the ones recommending an increase in complexity rather than those going by conventions. Keep your access openings optimal (not larger than necessary for excellent visualization and straight line access) and shave off time from the procedure while reducing the incidence of iatrogenic problems (file separation, ledging, and missing canals.) I just wrote an article in Inside Dentistry on the Role of Efficiency in Endodontics which I will add to the Article Section of the website now that this subject came up (read it in the Articles Section) You can review if for a big picture view of our ideas on this subject.
The attached picture series here are ones I used to demonstrated how my attempt to do a Ninja access allowed me to find the three canals, but caused me to miss the MB2, which I had to enlarge more to find. Thank you for your question.