That's well and good, but the reason that it is appropriate when using a pen on printed text is that the vast majority of people don't differentiate their penma

That's well and good, but the reason that it is appropriate when using a pen on printed text is that the vast majority of people don't differentiate their penmanship when representing emphasized text. Historically underlined content == actionable hyperlink; for content that is intended for public consumption (ie. not a note that you're writing for your own use) there are usability issues with typographic styles that represent emphasis with an underline.