In my opinion, I believe llvm-g++ is a more promising approach to supporting C++. C++ is an incredibly difficult language to parse, so why not re-use work that has already been done?
Because g++ has its own problems that won't be solved by switching backends, like unclear diagnostics. Then there's the license issue, which can be important for some. Clang is also built as a library, so you can link to it and JIT c-code at runtime.
Useful error messages. Even in C gcc is lacking, let alone C++, and I'd imagine that fixing this in gcc wouldn't be/have been much easier than writing clang.