This is one of the better threads I've read in a while. But I want to take it a step further. Crazyv correctly points out that primaries illustrate the breadth of a candidate's support and caucuses illustrate the depth of the support (an excellent and elegant way to describe that, which I will be appropriating for my own use!).

But there is another issue that needs to be illustrated, which is a candidate's ability to work with Congress and other members of the party. We've seen for 8 years the necessity of this. Bush gets everything he wants because his party in Congress votes as a solid phalanx and never betrays him. When Democrats are in office with a Democratic Congress...let's just say things don't usually work that way (see: 1993-1994). So it is critically important that a candidate be able to work with party leaders as well, and a demonstration of who can win more of that contest is also useful in determining who will be the best nominee.

Wait, did I just come up with a reasonable explanation for the existence of superdelegates?