Why would it be advantageous to have more or fewer bones in the pectoral girdle?

It is advantageous to have  lesser number of bones in the pectoral girdle.

Explanation:

Pectoral girdle is made up by the bones that connect the upper limbs to the appendicular skeleton  which includes all the limb bones. Pectoral girdle in humans is made up of two bones. They are clavicle (collar bone) and scapula. The pectoral girdle allows the free movement of upper limbs.

The bones of pectoral girdle are not as massive as the bones of pelvic girdle since it doesn’t have the function of support. Lesser number of bones in pectoral girdle makes  movement of upper limbs more free.

you did not attach the discourse, but i assume it to be the discourse on the origin and basis of inequality among men (the original titile in french is: "discours sur l'origine et les fondements de l'inégalité parmi les hommes"), also know as "second discourse". in this discourse, jean-jacques rousseau emphasizes that private property is the basis of all inequalities.

therefore, what enabled the old regime to maintain as long as it did was private property.