Ultimately, as I wrote with others some nine years ago, some practices
should not be codified. With IPv4 at least there was a plausible
argument for network 10. I didn't like it, nor did I agree with it, but
it was plausible. The same cannot be said for v6.
Incidentally, Sun & HP's use of default network numbers didn't really
cause any great consternation or motivation for RFC 1597, so far as I
could tell. Were that the case we wouldn't have needed either a /16 or
a /8.
Eliot