I can also use an extended link to model n-ary relations. XArc and RDF do not use different syntax for specifying a single arc depending on whether one of its end-points is inline or not. As to requiring a container just in case you want to lexically scope other arcs into the link, I don't think its necessary. In the case that you do want to elide the specification of one of the endpoints in a set of arcs that all have the endpoint in common, go for it. Altavista allows this also.
Authors decide what relationships they want to assert.
If I want to specify only one of those, it is also my prerogative. If someone else wants to assert relationships based on the relationships that I asserted, that is also their prerogative. I don't think so. I should just always use a general set implementation. Take this as an analogy rather than an invitation to go down yet another rat-hole.