The Conservative Purity Test

The Washington Post's Dana Milbank has performed yet another analysis of the so-called conservative purity test - the call by the supposedly "pure" members of the GOP to purge the RINOs.  Unsurprisingly, a high number of historical giants within the Republican Party failed to pass muster - including both Bob Dole and Gerald Ford.  Why is it that so many insist on purging the moderate and "liberal" Republicans from the ranks of the GOP?  Isn't the goal of a political party, particularly in a two-party system, to expand the tent to include as many allies as possible?  Even if one disagrees with a liberal Republican, it is likely that there is more ground for agreement with them than a Democrat.  Driving them from the party helps no one but the other side.  If the conservative members of the GOP really want a more pure (whatever that means) party, they're better off trying to convince these "tainted" Republicans of the rightness (pun intended) of the conservative viewpoint, thus keeping them in the fold.  After all, while ideology is important, in a democracy, numbers matter too.