I think you misunderstand what I mean by "prototyping or quickly mocking up a design that is changing rapidly". It has nothing to do with achieving a pixel-perfect design, finding the perfect shade of gray, or the right font size. It has a lot more to do with quickly testing ideas and iterating, rather than committing to a design and trying to get a finished look.

Paper does work well for this, and you're free to not like Balsamiq.