Protecting Yourself from HTML Snooping
From a security standpoint, anything that comes from somewhere else and runs applications on your computer, no matter how seemingly innocuous, is a threat, and should be avoided if possible. However, in many ways, the horse is already out of the proverbial barn here. HTML is, for the most part, probably here to stay as it is built in as a default "feature" to more and more email clients. It's also difficult to explain to novice users why their flowered stationary and HUGE BLUE FONTS WITH UNDERLINES AND EMPHASIS are not as good as plain old boring ASCII text where italics are indicated by _manually added pre- and postpended underscores_.
The best way to protect yourself from someone using this type of snoop is to use a non-HTML compliant browser. Unix-based mail readers such as MUTT, ELM, and PINE not only don't fall for this trick, but they're immune to the Microsoft-specific viruses that currently plague the Earth.