From economic sense it makes absoultey no sense to sell DP right now. Like I said, DP brings revenue on and off the pitch. Image rights, frendlies bla bla bla, all of those generate money because of the huge popularity of DP. These revenues more than cover his contract value. So when you get rid of him for a sum of cash, you have to remember you lose the DP revenue at the same time.
Sure you can argue that Cassano can bring in the same amount of revenue, but seriously, would Asians (those in japan) like Cassano (especially his looks) as much as DP???
Sure you get a sum of cash right now if u sell him off, but how much would you get ? Would it make sense to sell him for 10 million euros ? Considering the lost revenue, probably not.
Oh, and I think by using DP as a bargaining chip, we command more for each friendly in Japan. Hell Real Madrid earns like 10 mil more for each friendly .. or total, i forgot, JUST because of the presence of Beckham.