I think his idea was to save space.

yep.

partly it is also that some parts of my 3D engine are currently also based around using 64-bit tagged references (IOW: dynamically-typed data), and if a value can be fit into 56 or fewer bits, then it allows it to be stored directly in the reference without having to allocate additional memory.

granted, the usability of something like this does depend some on having usable numerical precision.