In its current state, default.txt doesn't allow us to customize environment variables that are set during build. For example if we had two identical builds where the other has python 3.5 and the other one python 3.6, we have no means of telling Coin about this change. Not to mention any way of doing the configuration itself.
With a yaml file we can define any data related to the build. So in addition to just configure arguments being split there, we should also tell every environment variable used. Why not the configure command, compiler command etc as well.