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

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".