Well, in practice, HTML in emails almost never is worth it, and that's not because richer markup is necessarily a bad idea, but because people don't grasp the concepts, and because what plain text provides almost always is actually powerful enough, while being a lot easier to use - which is important, given that emails tend to be read once, in contrast to websites, which tend to be read lots of times, so more effort in composing them is justified.
In order to compose an HTML email that actually takes advantage of the power of HTML, you have to grasp the abstraction between what you see on the screen and the underlying document structure, and how that underlying structure might be rendered by different receiving MUAs - something that most users don't grasp at all, and also a problem that's not particular to HTML email, but one that is well-known to affect all WYSIWYG authoring tools, and one that is known to be particularly problematic with document types that can be rendered very differently depending on the "medium".
By contrast, the typewriter style interface of plain text email composition is something that's completely intuitive to most people, and people easily succeed at composing an email that reliably will be rendered at the receiving end exactly as they expect - and most of the structure that you need for most email correspondence can actually easily be constructed manually from individual characters (headings, paragraphs, lists, numbered lists, ...).
Also, it can be observed that almost everyone reads books and journals typeset with TeX or DTP software. That's not exactly a good reason against creating your shopping list with a pencil.
In order to compose an HTML email that actually takes advantage of the power of HTML, you have to grasp the abstraction between what you see on the screen and the underlying document structure, and how that underlying structure might be rendered by different receiving MUAs - something that most users don't grasp at all, and also a problem that's not particular to HTML email, but one that is well-known to affect all WYSIWYG authoring tools, and one that is known to be particularly problematic with document types that can be rendered very differently depending on the "medium".
By contrast, the typewriter style interface of plain text email composition is something that's completely intuitive to most people, and people easily succeed at composing an email that reliably will be rendered at the receiving end exactly as they expect - and most of the structure that you need for most email correspondence can actually easily be constructed manually from individual characters (headings, paragraphs, lists, numbered lists, ...).
Also, it can be observed that almost everyone reads books and journals typeset with TeX or DTP software. That's not exactly a good reason against creating your shopping list with a pencil.