I think I may have manually created partial classes for huge classes to make them more manageable. Think for example expression tree visitors or collections of extension methods. Sure, I could just have created several classes like FooExtensions and BarExtension, but having a partial class Extensions seems ever so slightly better to me. But I generally agree, there are not many good use cases besides code generation and if you are tempted to do it, then you probably have a problem that has a different and better solution.