The .NET outputted binaries will still work on XP (they use the framework CRT, not the VS CRT) and you'll still be able to use old versions of Visual Studio both on and for XP. If you're targetting XP why do you need the latest and greatest compiler? VS2010 end of lifes a long way into the future.
I doubt that the .NET Framework 4.5 will support Windows XP when the VS 11 CRT will not. Admittedly, this is pure speculation.
There are a couple of reasons to use the latest compiler: C++ 11 (at least the library stuff has been implemented), C++ AMP etc. For .NET 4.5: async / await is an exciting new feature, that surely everybody wants to use.
Why should I be forced to wait for another two years before I can use these features in my products?