HTML email is a Bug, not a Feature.
1. Their bloated carcasses float around the internet carrying sickly sweet cows and hearts and colored wallpaper that add nothing to the message.
2. They are larger, and thus the more there are of them, the greater the impact to email servers.
4. Prevent my email from arriving sooner because my work server is clogged up dealing with their bloated carcasses.
5. See #1
Not that I feel strongly about this subject or anything...