I agree with mspguy.
In addition to his reasons, I think it would be difficult to find C++ educational materials (books, classes, etc) that teach it in a way that is relevant to small embedded systems like Arduino. A lot of "the power of C++" lies in its ability to draw upon a vast number of libraries and classes that implement "standardized" access to a lot of the things you want to do in a typical desktop environment, and a lot of C++ books will focus on that (creating windows, handling mouse events, interacting with the file system, etc.)