And why not use mingw gcc? Do they find somthing wrong with it? Edit Mingw-w64 its quite nice and includes gcc without all stuff cgwin does.
1) ABI compatibility is not present. With clang-cl you can compile half of your object files with MSVC and half with clang-cl, and link them together. That's very powerful. You might be thinking "but who is really gonna do that anyway?" However, there is more to the ABI than just this. For example, consider what happens if you need to use Structured Exception Handling. Well, with GCC, you can't. 2) Debug Information.…
You can on 64 bit.