Turn 100 developers loose and amazing things happen to your "source code" repository. I used to find entire images of Microsoft CDs in ours. The developers wanted to be able to get to known state for compiling, Visual Studio and anything else they were using went in to CVS, and later subversion. Then because they had so much trouble with different versions of the Visual tools making different output they started checking in all the built DLLs and executables.
The developers should have been soundly thrashed with a cane, but if it is possible to check in a binary. They will do it.
If you want to reliably maintain (or even build) a given version of the code, it's the right thing to check in the versions of the compilers and build tools that are known to work. A good tool is small, fast, meant to be run non-interactively, and takes instructions in an easily-reviewed file. Making your official build in Visual Studio is a bad idea for all these reasons--even Microsoft doesn't do that, at least when I contracted there. (They used a make clone with some extra support for metadata from win32 binaries.)
The developers should have been soundly thrashed with a cane, but if it is possible to check in a binary. They will do it.