I think you are right. The /MIR option says that /MIR is equivalent to /E and /PURGE, and /PURGE is supposed to "delete dest files/dirs that no longer exist in source". I have no idea why it isn't ...
source : Source Directory (drive path or \\server\share\path). destination : Destination Dir (drive path or \\server\share\path). file : File(s) to copy (names ...