I beg to differ. Last time I had to use PowerPoint (granted, that was ~3 years ago), math on the slides broke when you touched it with a client that wasn't of the same type as the one that initially put it there. So you would need to use either the web app or the desktop app to edit it, but you couldn't switch between them. Since we were working on the slides with multiple people you also never knew what you had to use if someone else wrote that part initially.