Yes, it is used by someone who compiles a program that does not start with the desktop version of the CLR. Like Silverlight, it targets the .NETCore or Micro Framework. They have their own mscorlib.dll, of course, using System.Object.
Here is the command line of the Silverlight project compiler:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Csc.exe /noconfig /nowarn:1701,1702 /nostdlib+ /errorreport:prompt /warn:4 /define:DEBUG;TRACE;SILVERLIGHT /reference:"c:\Program Files\Reference Assemblies\Microsoft\Framework\Silverlight\v4.0\mscorlib.dll" etc...
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