People entering college today have never known a world without an Internet. HTML was invented 1980, that's more than thirty years ago or 1.5 generations.
But plain text mails are still common despite all their problems:
- Encoding issues
- Wrapped code segments
- No links
- No way to use the "a picture says more than a thousand words" lore
Most of the security risks are now handled by the underlying browser engine and smart settings like:
- Don't execute attachments
- Don't download external resources (like web bugs)
On top of that, only very few people still read mail only in command line tools like Mutt. Knowing Mutt myself, I'm pretty sure you can configure it to display HTML mail with, say, w3m. On top of that, most HTML mail capable clients send two versions of the mail (pure text with an HTML attachment).
I'm not sure if there are any people left on the planet which still use a 56kbit modem to access their mail accounts.
So what reasons are left to stick with plain text mails in 2011?