Gyrus: if you provide a semantic markup language, they won't come. People won't just spontaneously write properly-categorized semantic markup because in theory are are useful distinctions to be made.
Though I suppose, given a semantic document source you could map it more accurately to HTML that made the distinction between i and em. Though I somehow doubt that this will be common enough to make i/em distinctions valuable for a screen reader. It's not that people who don't distinguish are insensitive to the blind -- writing relies on empathy, and there's simply a disconnect there.