I think the poster is quite correct- open primaries, closed primaries and caucuses all have a place in the nomination process.
A primary measures the breadth of candidates support a caucus measures the depth of support. Lets face it poll after poll shows that more people believe in the Democratic agenda (if not identified as the Democratic agenda) yet Republicans have consistently beaten Democrats. The simple answer is that their depth of support is much greater than that of Democrats. People who show up at caucuses are also the ones likely to show up to do the door to door canvassing, phone banking etc as opposed to the drive by primary voter.
We should also remember we are electing a PARTY nominee. The constitution makes no mention of Parties. So is it fair that those who are die hard party supporters who do all the heavy lifting have the same say as those who kind of show up on primary day.
Bottom line- all these different systems provide a different view of the electorate. The ideal candidate is one that can win all the different types of contests.