Prince of Tennis is obviously nothing more than a cash-in at this point.
I mean, the plot for this is pretty much 'Assemble ALL THE POPULAR CHARACTERS to fight against new enemies.'
Personally, I stopped caring about believability when it comes to PoT. I'm just watching/reading it for the hilarity of it... as well as the fact that despite how one-dimensional some PoT characters are, I'm a fan of some of them.