Value of backup MX

[John Goerzen <jgoerzen@complete.org>]

I'm looking at redoing my mail setup due primarily to spam filtering.
Over at http://www.tldp.org/HOWTO/Spam-Filtering-for-MX/multimx.html,
they are suggesting not to use redundant mail servers unless needed for
load balancing.

The last time I set up a major mail server, which was indeed a few years
ago, common sense was to always have a backup MX.  But then that was
before the days where joejobs, spam/viruses with forged headers, etc.
were popular and troublesome.

Now, I'm not really an ISP (this was the closest list I could find for
this topic), but I do host a number of mid-size mailing lists and also
receive a good deal of mail myself.  I'm wondering what your thoughts
might be on no longer having a backup MX outside my direct
administrative control (or even one that is within my administrative
control).

It seems to make a lot of sense to me, but it seems too that I must be
missing something.

Thanks,
John