[Exim] Why is $recipients forbidden ?

I wanted to use the $recipients expansion variable, but it is
forbidden outside of the system filter "to prevent exposure of Bcc
recipients to ordinary users".

Is it so that if a delivery is made to multiple local users, that
$recipients as it is now would contain all of those users when
the user-filter is run? In that case, it makes a bit of sense.

But wouldn't it be a good idea to lift this restriction when
a transport is run ? That way, I could do something like:

headers_add = "Envelope-to: ${sg{$recipients}{, }{,\n }}\n"

in a local transport. This is for storing all messages for a
domain in a maildir mailstore, *after* alias processing etc.
By using the standard envelope_to, I'd get the addresses before
alias processing, but that's not quite what I want.

I'm assuming here that $recipients at local-transport-time is
just the list of addresses passed to that transport, right ?

Mike.
--
"Insanity -- a perfectly rational adjustment to an insane world."
- R.D. Lang