Why should I write CLS compliant code?

If you write a library or framework it makes sense to ensure your library can be used from any CLR language.

CLS-compliance is particularly important if you're distributing libraries - specifically, writing CLS compliant guarantees that your libraries will be usable by all CLS-compliant languages.

For instance, Visual Basic is not case-sensitive, whereas C# is. One of the requirements of CLS compliance is that public (and protected) member names should not differ only by case, thus ensuring that your libraries can be used safely by Visual Basic code, or any other .NET language that doesn't differentiate based on case.

The answer is to allow maximum compatibility across .NET languages. CLS is the lingua franca that allows C# assemblies to work with F#, Iron Python, C++/CLI, VB.NET, Boo and all the other .NET languages. Step outside that boundary and your assembly may work correctly, but not necessarily.

There may not be a specific reason to have your code be CLS compliant, but people are referring to it being a "best practice"--something that you should do because it's a good habit, rather than being measurably better for a particular scenario.

In other words, it's a good idea to make your code CLS compliant unless you have a reason not to.