Info =/= text, so therefore I'll read all the HTML content. Heck, I'll read both to see the underlying code if needed.
I use plain by default and if it's trusted/legit I will switch to html if needed. I hardly ever need to.
HTML email can present security/privacy risks. Non-embedded content can be changed or removed after the fact which makes such messages less reliable and more difficult to correctly archive. More than a few senders of such are lazy and either don't include a text/plain alternative or simply make the text/plain alternative "Click here to view this message". The former encourages people to enable something that there is good reason not to enable, and the later encourages "just trust email messages that appear to be from us and click through" behavior that can make people more susceptible to phishing. Often those "Click here to view this message" links are HTTP and sensitive info would be passed in the request or response. Even in cases where the email was delivered via STARTTLS. It is surprising how many companies, including large bulk emailers, broadcast "we can't even do email correctly so don't trust us to do anything else well" to the world.
The only way to get infected via an email is from attachments, which people usually ignore, because of well known risks, or through HTML, needless to say more?!
HTML/Rich text messages allows also to track you by using embedded in email transparent GIF one by one pixel graphic. Of course if you have enabled displaying images. This practice is often in use by marketing companies but anyone can use it too.
I'm using HTML in Thunderbird as scripting is always disabled and external content is blocked by default. Thus, I don't worry about security or privacy problems.
Full HTML, but I always get text-only based emails so I'm starting to wondering if I actually need full HTML view.
My Gmail is set up to accept HTML from trusted senders. I'm aware there is some risk involved with HTML, but I can also tell you it gets old pretty quick to have "image blocked" splattered all over your emails.