I was always a strong advocate for plain text emails. Because for many years my mail client was Mutt, I saw everything plain text. Now that clients are graphical, it bothers me to know others might be reading my plain text emails with a "Times New Roman" font, and not a monospaced (as god intended it to be).

So, to avoid that, I use HTML on my emails. Yes, it makes them a bit bigger. Yes, they go against my very belief. Yet, they look like I want them to be seeing.