I agree with the sentiment behind this post. If Ron Paul ran as a third party candidate - he'd take more votes away from Barack Obama than from Mitt Romney. The Occupy Wall Street, the anti-war crowd and those generally disappointed that Hope and Change resulted in more of the same would find a voice for their protests in Ron Paul. Those people make up some of the very base Obama has been pandering to.
The Republican base and the Independents worried about the economy aren't going to forsake Mitt Romney for Ron Paul. Giving up the fringe to get the center is a very good trade for the presumptive Republican nominee.
A third party run by former New Mexico Governor Gary Johnson would also hurt Obama much more than Romney. Johnson on the ballot would attract people whose main issues are the environment or legalizing drugs. Correct me if I'm wrong but I think those people voted for Hope and Change the last time around.