The NDA is in fact an alliance of a "Communal" led BJP of "Secular Parties. The UPA is an alliance of "Secular" Congress many of its members where formerly part of "Communal" BJP led NDA.
So knickerwallahs crying opportunism in SP-Congress link up is in fact smacks of hypocrisy.
Unless we give a single a party a majority, these kind of alignments are something we have to live with it.