We had a problem at work that caused our .NET 4 application to fail on .NET 4.5 and that forced us to include .NET 4.5 in our testing phase.
We have no intention using any of the features in .NET 4.5 anytime soon but unfortunately that doesn't prevent our application failing on a users machine if they have installed .NET 4.5 even for previous versions of our application!
The other side of the problem is that we can't make our users stop using Windows XP so we have no good reason to stop supporting it for a long time to come.
So in conclusion, I don't really care that .NET 4.5 isn't supported on Windows XP but I hate the fact that it's an in-place upgrade to .NET 4.0.
Why in the world would Microsoft do this? The only reason I can think of is so they can phase out Windows XP and sell more copies of Windows 7 and 8. At the cost of developers all over the world.