Pre-increment/decrement can be more efficient than post-increment/decrement. Post-increment/decrement usually involves keeping a copy of the previous value around and adds a little extra code.
Xerces-C crashes when a file name is NULL. If the locale is wrong, wxString::mb_str returns NULL.
Wide file names also enables opening a file with any characters in its name on Windows. Since there is no wide file names on POSIX, it doesn't make much difference except that the program is more stable.