Romney may have already promised Paul that he will audit the Federal Reserve or presumably something else. Notice that during the debates Paul and Romney never attacked each other. http://www.nytimes.com/2012/02/17/us/politics/mitt-romney-and-ron-paul-friendly-amid-the-rivalry.html?pagewanted=all
Forrest Gump - "A promise is a promise Lt. Dan."
But we would still prefer to have him in the cabinet.