The question has been posted at Stackoverflow where it was suggested that this could be a potential bug.
Stackoverflow link : http://stackoverflow.com/questions/13643700/complex-table-formatting-in-restructeredtext
Attaching the source rst files : index.rst and chapter4table.rst and the tex output :
rst file containing the complex table
This seems to be the known bug described in http://docutils.sourceforge.net/docs/user/latex.html#tables
multirow start line ends with %
the bold (textbf) formatting start therefore was commented out