I know that .Net provides the feature to mix several languages within the same application (there are some constraints of course, but this is not essential). Theoretically, this might be done because the languages supported by .Net do not fully expose all the features it can provide. IL does of course. So when you need a feature that the current language doesn't provide, you could write it in another .Net language. The whole code would be translated into IL, and so you would gain more features. 

From my personal experience, I was writing a project in C# and needed to integrate a small project written in C++ under .Net. When I consulted with some more experienced programmers, they said that it would be much easier to "translate" the C++ code into C# than integrate it. Since I had little time, it was what I did, and it was quite easy (the project was quite small).

So my question is, has anyone experienced a real-life situation when mixing languages is an absolute necessity?