When you say you think it is very useful, do you mean that it's useful
for NetBSD to mimic other systems, or do you mean that it's useful in
some other sense?
Copying directories recursively preserving file attributes is one of
the file operation I do regulary. So, -a option seems to me a little bit
more convenient to type than -Rp (-P is not mandatory as it is the default)
and easier to remember (<A>rchive). Also -a may be helpful for those
who come to NetBSD from other systems, notably FreeBSD and Linux.
So, yes, mimicing other systems may be a benefit too taking into account
that 'cp -a' is accepted in Linux and some other platforms
for very long time.