It is supposed to advance the conservative cause by sending the lunkheads who run the RNC and similar organizations a very pointed message - stop sending us RINOs/faux conservatives to vote for.
I think the problem is that most Americans aren’t super conservative. They’re more middle of the road. It’s looking to me like many in the far right have decided they will not be happy without a far right candidate, which we’ll never have in the Republican party because far right candidates don’t appeal to a broad enough base to get elected to the highest office of the land. I think it’s unlikely we ever have a Republican nominee conservative enough to satisfy all the people saying they’ll stay home or vote third party this year if McCain ends up being the Republican candidate. I would suggest that those who would stay home or vote 3rd party need to form their own party so they can nominate who they want to nominate. They’re party probably won’t have any better luck than the Libertarian party at getting candidates elected, but at least they won’t have to sully themselves voting for someone not conservative enough for their tastes.