[TeMPOraL]

Underlining also represents the thing you often do with a pen on printed text. It's perfectly OK and useful type of formatting online.

[Mtinie]

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.