The problem with this is that the express editions are a stepping stone for new / younger programmers (and for devs wanting to contribute to OSS). The harm that it'll do will be to the whole .NET platform (and the ability to find good .NET devs in the mid-long term), which would overshadow any benefits of people moving to MonoDevelop or similar (they're more likely to just move to other languages).
Not sure what Microsoft is trying to achieve by this - it's a bit of a shift from their previous stance of giving as much stuff to developers as possible, using that to prop up the windows ecosystem.