libxml2 and our XPATH function doesn't support default namespace ( http://plasmasturm.org/log/259/ ). This is pretty useful feature - so I implemented. This is the mayor issue of libxml2 library. Another difference between XPATH function and XMLTABLE function is using two phase searching and implicit prefix "./" and suffix ("/text()") in XMLTABLE. XMLTABLE using two XPATH expressions - for row data cutting and next for column data cutting (from row data). The our XPATH functions is pretty simple mapped to libxml2 XPATH API. But it is not possible with XMLTABLE function - due design of this function in standard (it is more user friendly and doesn't require exactly correct xpath expressions).
libxm2 doesn't support xpath 2.0 where default namespace was introduced.