Why $rsyslog::params::rsyslog_d gets purged?

On Debian (squeeze) when installing other packages (like postfix) some of them add files to /etc/rsyslog.d/

In particular I can't see how to add postfix include to the mix of this module without coupling it with others or coding a specific hack on template.

Postfix include (/etc/rsyslog.d/postfix.conf) looks like:
~~~
# Create an additional socket in postfix's chroot in order not to break
# mail logging when rsyslog is restarted.  If the directory is missing,
# rsyslog will silently skip creating the socket.
$AddUnixListenSocket /var/spool/postfix/dev/log
~~~