Maybe I'm misreading the author, but I take "static allocation of disk space to support multiple HFS+ instances" as a fancy way to refer to multiple partitions on the same physical volume.

(So, Yosemite on one, and a pre-release of El Capitan on the other.)

Space sharing just promises to allow the same thing but without wasting space.

This seems too obvious and important a use case to call "specious and an uncommon".