"... Many - though not all - of the realistic cases of government activity can be fruitfully analyzed as some kind of a blend of these two extreme polar cases" (p. 350) - mixtures of private and public goods. (Education, the courts, public defense, highway programs, police and fire protection have an) "element of variability in the benefit that can go to one citizen at the expense of some other citizen" (p. 356).
From Pickhardt, Michael's paper titled "Fifty Years after Samuelson's 'The Pure Theory of Public Expenditure': What Are We Left With?":
"... It seems that rivalry and nonrivalry are supposed to reflect this "element of variability" and hint at a continuum of goods that ranges from wholly rival to wholly nonrival ones. In particular, Musgrave (1969, p. 126 and pp. 134-35) writes:
'The condition of non-rivalness in consumption (or, which is the same, the existence of beneficial consumption externalities) means that the same physical output (the fruits of the same factor input) is enjoyed by both A and B. This does not mean that the same subjective benefit must be derived, or even that precisely the same product quality is available to both. (...) Due to non-rivalness of consumption, individual demand curves are added vertically, rather than horizontally as in the case of private goods".
"The preceding discussion has dealt with the case of a pure social good, i.e. a good the benefits of which are wholly non-rival. This approach has been subject to the criticism that this case does not exist, or, if at all, applies to defence only; and in fact most goods which give rise to private benefits also involve externalities in varying degrees and hence combine both social and private good characteristics' ".