Too many purchasers of airplanes (i.e., airlines ... and others) use the “expertise” of those they employ or others on their payroll in one manner or another. Often it is the additional expense of including a direct reading AOA indication, together with the “advice” provided by these “paid consultants” (some of whom don’t know their knee from their elbow and others often don’t want to “rock the boat) who generally advise that obtaining such an AOA indication would be “nice,” but it does cost more. Inevitably, a good share of these airplane purchasers decide to forego the “additional expense” since it would be merely “nice to have.” Anyone who has operationally flown airplanes with direct reading AOA indicators will tell you that it is a very cheap addition that provides a wealth of knowledge to the flight crews … but, unfortunately, the numbers of these folks is not growing and they are not usually in the chain of advisors.