After installing libreoffice-bin, any plain text files and even c sources are opened by libreoffice writer by default. I'm using xfce desktop and have gedit installed.
I think libreoffice writer shouldn't be associated with plain text files, despite that fact that it can open them. Usually there is a more appropriate editor for plain text.
You will need to tell XFCE what to open certain filetypes with - this has nothing to do with what libreoffice says it can open.
*** This bug has been marked as a duplicate of bug 449958 ***