>Why would we want to exclude HTML and attachments?

I would like to answer this question in two ways.  First, in a high-pitched
squeaky voice, and second...no, never mind.

1) If you mean, "Why should I exclude HTML and attachments", mainly it's
for aesthetic reasons, so if you don't feel the need don't worry about it.
For more info see below.

2) If you mean, "Why would _anyone_ want to exclude HTML and attachments",
there are several reasons:

 a) HTML messages look really ugly to people who don't use an email client that
 supports HTML.

 b) HTML messages take up more space and bandwidth than plan-text messages.
 The former can particularly be an issue if you keep archives and have a
 limited amount of space (or are paying for your space based on how much
 you use).  Attachments similarly take up a lot of space and bandwidth and
 can screw up digests.

 c) Extremely large messages can cause problems for any of the machines
 they travel through.  Recently I had to help a user whose email client
 locked up whenever he tried to check mail; the problem turned out to
 be that someone had sent him four email messages, each with a huge
 file attachment.

I think that more-or-less covers it.  If not I'm sure others will fill in
the gaps.

-jwgh