His cash advantage isn't what it used to be. Time was that his moneybombs filled an account seven, eight times larger than Rick Santorum's. But Ron Paul has enough money to go on the air in Michigan, and this is the expense.
Michigan is a proportional representation state, so Paul can pick up a few delegates even if he dive-bombs into fourth place. The larger relevance of the ad, though, is something explained best by Amy Gardner.
Romney’s aides are “quietly in touch with Ron Paul,” according to a Republican adviser who is in contact with the Romney campaign and spoke on the condition of anonymity to discuss its internal thinking. The two campaigns have coordinated on minor things, the adviser said — even small details, such as staggering the timing of each candidate’s appearance on television the night of the New Hampshire primary for maximum effect.
In around 34 hours, Romney and Paul will share a debate stage with Santorum and Gingrich, both of whom have to attack Romney and don't have any competitive interest in Paul. He'll be busy; he'll be Romney's sole ally up there.