Funny thing is that internal to Microsoft, most devs don't use Visual Studio as an IDE. Maybe as an editor or a debugger, but there are a lot of build systems out within Microsoft and most of them don't plug into Visual Studio.
IMO, open sourcing Visual Studio itself is not all that interesting vs. open sourcing the .NET platform.
I worked at Microsoft Excel 15 years ago and Visual Studio was only used as a debugger. Visual SlickEdit was the editor and compiling/building was command-line based. Each group is different.
Ah, that may be true. However, there was a push within my division to standardize all of our tools with VS (there used to be a separate editor for X++ but now it's all VS), and I've heard other things about other divisions.
Source is I worked there for eight years. Left two years ago. There was a push around the time I left to standardize more on TFS, but historically, VS was regarded internally as a bit non-hardcore.
IMO, open sourcing Visual Studio itself is not all that interesting vs. open sourcing the .NET platform.