I am writing both the "optional" ELEMENTS and VERTICES sections in my unstructured mesh (along with the required CELLS and CONNECTIONS blocks), and I have the PRINT_PRIMAL_GRID flag in the OUTPUT block, but I still don't get any xmf files.
I have a python script that creates them, so I have a sort of work-around, but it would be nice to have this work correctly.
This came up last year, and there was some discussion about how to possibly fix it, but I am not sure exactly what happened.