For hardcore techies who still ssh into their boxen to check their email, this may be understandable. But for people using regular mail clients on GUI-based systems, this is nothing short of "ideological-bordering-on- stupid". There is a reason for email - communication. At the office, it is meant for getting work done.
So, if you use HTML mail for putting some highlight or emphasis in your email that will make your recipients' job of sifting through all their email easier, go ahead and do it.
I personally receive all my work email in plaintext (there is an option in MS Outlook which allows you to do that). A nifty option appears on top of my message window, which allows me to see the email as HTML. For most emails, where formatting is just eyecandy, I get by with plaintext. But for inline answers to emails, color contrasts are extremely useful. Then of course, there's tables, which are great for some of the work we do.
Being ideological about tech issues is good. Being an ass about them for the sole reason of ideology is not. Standards were meant for making things easier for us. Not the other way around.