Okay, correct me if I'm wrong here:
So if I want to write an app that uses HttpUtility.HtmlEncode or UrlEncode, the user needs to install the full framework, or I need to roll my own.
A lot of users will be installing .NET twice per version. I can already hear tens of thousands of people screaming "but I thought I already installed .NET 4".
I honestly fail to see how this is a good thing. More versions of .NET for embedded/mobile OSes would be nice, but more versions for Windows just seems silly.