Yes, let's be practical. Why are we conservatives expected to throw in the towel some two years before the 2008 election and agree on a liberal presidential nominee? It ain't gonna happen! Millions of conservatives will sit it out or vote for a third party, rather than turn over the reigns to liberals.
But that is exactly what they would be doing by voting third party or sitting it out. They'd actually be assuring that the most liberal one won.