I agree. The data validation example doesn't seem contrary to the advice; you would generally have at least three data types you need to handle in such a case.

I agree. The data validation example doesn't seem contrary to the advice; you would generally have at least three data types you need to handle in such a case. The difference with the dialog window is that (presumably) you don't know the different flavors of window you'll need to render so adding an abstraction on top of the existing rendering abstraction fits squarely in "premature optimization".