When opening a document MS Word copies it to temp file and then renames the latter into the original name. Original file is deleted.
When opeining Word document on CIFS folder MS Word creates brand new document. And original document is deleted including version information.
It's fairly obvious that is not an Alfresco Team's bug, but I'm afraid there should be an effort to work around this situation. Otherwise CIFS becomes useless - users have to keep in mind that they are to upload Word documents and they are able to edit other formats in place.