Did they teach you in high school the relevance of that history? Does the fact that it'd been traditionally used as a substitute mean you can't use underlining now, so it should not event exist in markup?
It's pretty obvious that underlining is something that rarely appears in print. It is mainly an annotation device: something added to finished print, by hand.
When was the last time you saw underlines in a book that weren't added by a previous reader?
Underlines are bad typography that should be used sparingly, if ever.
It's even more obvious that we're not talking about print, but a markup language anyone can outside of book publishing. So why do you want to prevent a "previous reader" from using said annotation device for the little note he's creating/editing?
We got here because you mentioned all the irrelevant stuff your learned in high school, and when I pointed that out, you mentioned some more, now about the whole underline, which I used as an example of how there is no intuition behind _ since that intuition is "taken" by underlines (that's why you needed to criticize underlines)
When was the last time you saw underlines in a book that weren't added by a previous reader?
Underlines are bad typography that should be used sparingly, if ever.