Pro Powerful language
You can write code for your build system in Ruby. While not my choice for general programming, Ruby is powerful and expressive. Given some knowledge of Ruby, you can create powerful Rake extensions that result in your average target only needing a few lines in the rakefile in spite of having complex behaviors (Is the library for public consumption, or only for use within the current repo/tier? Compile certain files on certain platforms? Link to libraries published from other repos? etc.).