Here is a possible motive for McCain's game I hadn't thought of :
McCain's ploy was transparent. To counter the public's preference for Obama's economics over his own, he would get both of them in a room and emerge proclaiming that they had reached agreement, that they had no differences. In fact, they have very real differences. McCain wants to retain tax cuts for the wealthiest Americans; Obama wants to create tax cuts for all but the wealthiest 10 percent of Americans. Obama favors policies -- through investments in infrastructure and education and through legislation enabling Americans to join unions without fear of being fired -- to build the base of the economy, while McCain's record is one of opposition to such policies. Obama favors trade agreements only when they raise labor and environmental standards with our trading partners and protect them here at home; McCain has supported every trade pact that has weakened such standards and has never said one word about protecting our standards or raising them abroad.
When McCain wanted to hold all these town hall meetings, we figured it was so McCain could surround himself by a large Obama crowd. Now it looks like McCain is trying to leech off the image of Obama again.