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.