[Exim] Exim4 + SpamAssassin

Hi,

I have been looking at several ways to incorporate SA system wide. One of
the ways I have seen several times, is the use of a spamcheck transport (and
router):

spamcheck:
driver = pipe
command = /usr/sbin/exim -oMr spam-scanned -bS
transport_filter = /usr/local/bin/spamc
bsmtp = all
home_directory = "/tmp"
current_directory = "/tmp"
user = exim
group = mail

So the mail is put through spamc and the resulting output is piped to Exim.

Now I was thinking: wouldn't it be easier to just use transport_filter on
existing transports? I guess I must be missing something. Please enlighten
me :-)

local_delivery:
driver = appendfile
delivery_date_add
envelope_to_add
file = /var/mail/$local_part
return_path_add
transport_filter = /usr/bin/spamc

Would this work?

Best regards,
Martijn.