It might prevent viruses but it probably won't prevent corporate espionage. Viruses usually try to control the system for which they need admin privileges (unless a bug is used in the OS). An infected pdf file that you got through email would make broken pdf reader steal documents because under current security models (in Windows, Linux or OS X)  documents usually aren't protected from processes running in medium IL, unless special care is taken. If WinRT applications are allowed access to any of user's files without OS interfering with a permission window (similar to UAC) or WinRT allowing access to protected files only through OS provided file Open/Save file dialog then the sandbox protects only the system and not documents.