(Hey, a meaningful subject line!)

> > The two structures (rc.local vs. init.d) can each emulate one another. 
> No they cannot.  rc* can be emulated by 'init.d', but the reverse is
> _not_ true: any worthwhile init.d implementation provides the
> capability of performing "shutdown" actions, and that's just _not_
> present in rc*.

Well, you could (in principle) have init run /etc/rc.shutdown as a logical
extension, but now package designers get to design sed scripts to damage, er,
update TWO customized shell scripts instead of one.