A couple of notes on this. Firstly, it's not actually new. The term has vascillated for years. Spark uses the singular, for example. The Forerunner civilization self-identified (not unlike Iain M. Banks' Affront
http://en.wikipedia.org/wiki/List_of_societies_of_the_Culture_setting#Affront) as "Forerunner" - often in the singular as a proper noun, as a somewhat dishonest self-identification as a "stage" in Galactic evolution. Of course the majority of them had no intention of going anywhere.
So in the past tense, when that proper noun has become a descriptor of chronology, the plural starts to creep in. It is deliberately fuzzy and one of the other reasons we chose proper plural was to avoid the semantic awkwardness of hearing a modern human ignore the plurals, past-participles etc. Don't want Marines talking like they are larping at ComicCon.
Glaven.