Images can be attached to an email without HTML and URLs that appear in text/plain emails are "linkified" by most readers.

The original MIME specification included an "enriched text" subtype which was essentially an HTML subset that was easy to implement safely and served most purposes for person-to-person communication.

MIME allows absolutely anything to be embedded in an email, but the standard explicitly discourages the use of HTML as the message body. The argument for standard compliant HTML support in all email clients is fairly weak considering it shouldn't be there at all.

How do I send email in 24 different colors with pictures of my puppy, then?

A more serious restatement of the joke: Other than HTML email, there is no functional way for the average computer user to send personal information with the following properties:

1. Rich text with embedded images (you know... HTML)

2. available at work without running into firewall or security restrictions (Websense)

3. Easy enough to compete with email.

4. Cheap enough to the end user to compete with email.

5. Secure enough to allay concerns about personal information making it out into the wild. (Never mind that email is easily interceptable: there is a perception versus reality problem here.)

A web application isn't going to do it. It's just too painful to attach more than two photos. A separate application isn't going to do it -- what corporate IT department is going to install it?

Livejournal comes close for tech-savvy people, but it's not in the average user's current skill level.

It's a fundamenally hard problem for social rather than technical reasons, and that is why people use HTML in email, and that is why vendors support it. Reducing it to a technical problem doesn't help.

Some protocols cannot do certain things. This is an unfortunate fact of life. If you find an ad-hoc workaround, don't be surprised if you have to jump through hoops to make it work and don't blame someone for breaking it when it was never supposed to work in the first place.

But the situation is more like me using a hammer to knock screws in and then screw makers changing the screws design so that it's harder to knock them in with a hammer because "they were never designed for that".

The problem isn't that some protocols can't do these things. The problem is that email is the only protocol that fills this need, so it's time to stop complaining about how people choose to use it.