There is a class of people, which i'll just call unix morons, who abide by the creed that email should always be in plain text.
In the older days, say 1995, where the web is not as advanced as today, and the network is much lower bandwidth than today's, there is a genuine concern about sending emails that's more than few kilobites in size. In those 2400 baud rate modem days, many people have to pay for bandwidth byte by byte or minute by minute. Sending a attachment to a mailing list is one way to anger all the members. In the Netiquette FAQs of that era, there's even a warning that by convention signature should not be more than 4 lines. One of the other important reason people don't like images or attachment or enhanced texts in emails, is that there are newbies who have little understanding of writing or computer but just discovered the electronic mail technology in which he can send images or make fantastically garish visual adornment of his typings, often will do so just because he can, and without knowing why people didn't like his enhancement of communication.
Besides the afore-mentioned 2 reasons, there is one more: unix. The unix operating system, is so-called text-based OS. Every thing is a text file, from configuration files to IO. This may be a fine approach of things, but for reasons i won't go into now, the people who unix attracts and attracted-by tend to be happy-go-lucky sopho-morons. Together with unix, these sopho-morons developed a back-water philosophy towards technology. (one of which is why change when things ain't broken, see my other recent essay on that) Because unix is “text-based”, these unix lovers developed the sense that everything in the universe must be text-only and command-line driven. For example, the lip-reading computer program HAL depicted by Stanley Kubrick's 1968 film “2001” that is supposed to be available in year 2001, would be a egregious User Interface f���-up with respect to these unix loving people. Any technological advancement moving away from command-line will be ridiculed upon by these folks. Even today, we can still see quite a few of these command-line-supreme heads, and many traces or artifacts of command-line/pure-plain-text based softwares, such as CVS, Lynx, Apache, and the influence even extends to design of computer languages (such as the train of utterly idiotic imperative languages the likes of C, C++, Java, C#, perl etc. ( I will discuss these in the future.)). Notice the fantastically stupid quoting convention we use today in emails, by hardcoding a “>” prefix to lines, is also originated from the unix text-based mind-set. (from the convention of the second line shell prompt.)
The effect of these unix moron's plain-text and command-line mind-set are so sweeping such that even today in the year of our lord two thousand and two, in discussions or debates involving computer-human interfaces, that even intelligent beings who knew that graphical and other human interface technologies are clearly superior, but will sometime utter things like “different approach for different tasks” so as to avoid being snickered upon by colleagues as if he isn't manly enough to advocate the difficult command-line interface.
Today the web and bandwidth are vastly expanded as a result of commercialization, and netiquette FAQs are pretty much a historical relic. The restriction for plain-text only format in email is no longer a necessary practicality. The question becomes a matter of choice or philosophy.
Personally, i'm of the opinion that online discussion as of today should exchange in as much plain text as possible. Not that i oppose technological advancement, but subtle ramifications i imagine. Just look at our mailing list. There's already huge traffic, most of which banal. Now, if we suddenly encourage attachments or fancy formatting, then drivelers's drivel will jump out and hurt our eyes, burden our disks, jam our network, and irritate our minds. Just imagine each person will have his say about his choice of colors and background and fonts and soon it will be politically incorrect to criticize folk's email “skins”. Perhaps also because i'm kind of a conservative in certain matters, that i think the best written communication is unadorned with formatting fluffs or picturesque smileys. At the back of my paranoid mind i also fear that because technology is changing so fast such that fancy texts of today won't be recognizable tomorrow. I want to be able to show my rants to my grand-grand-kids when i'm 100. Perhaps winning their smiles.