If you don't check in the generated code, that means that you have to run the generator every time you build (or at least every time you get a clean copy of the tree and build). For code that changes very rarely, that may not be a net win.
Also, if you don't check in the generated code, and then you upgrade to the latest version of the generator, surprising things can happen (or even if different people have different versions of the generator installed).
So there can be cases where checking in generated code can be at least a reasonable thing to do.
