The principal value of the interrobang is that it looks more professional and correct than putting two different full stops on the end of one sentence. Unfortunately, it isn't easily differentiated from either of it's parents and lacks the impact of the !?, which is more often used in comics and advertisements, anyway and as such doen't need to look correct.
All told, I can't see much argument for bringing it back, beyond on stenographic keyboards where it's going to save your overtaxed court stenographer precious keystrokes. Notably, the courtroom is probably the place where emphatic questions are most asked. Alas, court stenography is about recording what was said, not how.
However, the interrobang does have one crowning virtue: It allowed me to feel good that I knew it was a variant of exclamation point because of the word bang in the name--no description necessary. This is of course, only because of my studies of programming and internet history, rather than any understanding of typesetter culture, but I still feel good about knowing one more useless thing.
