You can use ILMerge:

ILMerge.exe Foo.dll /ver:1.2.3.4 /out:Foo2.dll

A valid reason to do this is to increment the assembly version in a build in you find breaking changes (using NDepend for example). That way if there are no breaking changes the assembly version stays the same, and you can patch released builds easily.

We always increment the file version, and that reflects the build number.