One way to explain the creation of a nominative and accusatory case marking system is from an optimal theoretical point of view. Case marking performs two functions or limitations: an identification function and a distinguishing function.  The identification function is presented in an exemplary manner when the case morphology code (identifies) specific semantic, thematic or pragmatic properties on the nominal argument. Accusing cases in the position of the direct object, for example, can be a powerful identifier of the patient population. The distinction function is used to distinguish between central arguments, the subject and the subject of a transitory clause. Helen de Hoop and Andrei Malchukov explain in Case Marking Strategies the motivation and necessity of the distinguishing function: Nuger, Justin. 2009. On the declining existences and marking of the differential object in Palauan. In Austronesian Formal Linguistics Association (AFLA) 16, eds. Sandra Chung et al., 137-151. London: The University of Western Ontario.
Ruth Kramer. 2014. Double climate or object agreement: The view of Amharic. Natural Language and Language Theory 32: 593-634. I refer the reader to Rezac (2011) for the details of a cyclical-obedient approach to P0-Insertion, as well as the discussion on other environments where exceptional licensees occur. It is rare for the case to serve only the function of distinction, which overlaps strongly with the identification function. Other ways of standardizing the arguments of a transitive predicate (subject agreement, limitation of the order of words, context, intonation, etc.) may explain this linguistic observation. De Hoop and Malchukov argue that case systems entirely based on the identification function must be richer in case morphology than languages based primarily on the distinguishing function.
Josephs (1975) provides an alternative explanation of the demoted example (61b) with regard to the treatment of sentences. He notes that the presence of a single name a hong `a book` immediately after the 3rd man person at the plural point of arrangement is difficult to analyze. The assessment of the validity of this alternative must also be left to future work. Bhatt, Rajesh. 2007. Unakkusativitet und Falllizenzierung. Lecture at McGill University. Aissen, Judith. Marking the differential object: iconicity in relation to the economy. Natural Language and Language Theory 21: 435-483. Rodriguez-Mondoédo, Miguel.
2007. The syntax of objects: accepting and marking the differential object.