[Quoth]

In general a totally optional item a spec is often one that gets left out if any of the following apply:
1) Implementation spec writer can't see the point of it.
2) It's awkward to implement, or more work.
3) Project timescale is ambitious (every SW project I ever worked on in 30 years)
4) The other guys never implemented it
5) It was never used by other people that did implement it.
6) It was implemented, but hardly anyone tried using it, so we never fixed the bugs, doesn't matter as it's optional.

Item (6) does seem to apply to some epub implementations as putting the tag simply moves the item to the end rather than making it only accessible via links in other pages (or possibly the TOC).

So it's in the spec, but in a way that ensures it will either not be implemented or not as intended (like the mpb:section no cross tag). Having it in the spine does sort of make more sense than a fake HTML tag in the content, like mobi has.