The emdash is supposed to add emphasis much in the way people use air quotes.

As an English student who had to analyze sentences, I really disagree with the semi-colon. Semi-colons join two independent clauses that are contextually related. An Emdash replaces many punctuation marks like commas, colons, and ellipses. The emdash then could be joining a phrase, preposition, interrogative, dependent clause, or independent clause. Which means I now have to parse through what is meant to determine the structure. When a writer is sloppy with modifiers or prone to run-ons then it is vastly annoying to read and edit.