On further reflection, I realised that there is more to this than simple identification of the head noun. For example, both of the following sentences are perfectly OK, even though, strictly speaking, couple and pair are singular nouns:
A couple of birds were flying past.In fact, there are quite a lot of examples like that. In the following two sentences, crowd and majority are singular nouns, but there seems little problem in treating the subject as plural:
A pair of swans were swimming in the river.
A crowd of people were singing.Maybe the possibility of treating phrases with couple, pair, crowd, and majority as plural is nowadays being extended to use. So perhaps treating "use of these phrases" as a plural subject is not an error at all.
The majority of the people were happy.
Maybe this is an area where English is undergoing change. And perhaps this is one more area where my students are ahead of me in reflecting the ways the language is evolving.