No one has given me a straight answer. It seems that once upon a time they would have been useful for quick reference and for modulating large project. But now with OOP modulating is impossible to avoid (Java doesn't even have header files and Cocoa discourages them) and API documentation is done for us by Eclipse, XCode, .NET...

Is there any reason to still use header files?

Don't yell at me for doing poor programming, in C and C++ I do use them (for my own code, other files I often import as frameworks)