I believe the ones that do a responsive.css apart are missing one point when working with teams. In the company I work we do the approach you wrote... why? Because it's easier to know where something is there, which styles you've modified, and specifically know if you did a little hack or modification to bootstrap class but specially we do it like that 'cause we minimize git errors.
When someone is working on the index and other team member on the contact if we had a responsive.css we'll get conflicts every time we did a pull and push. And with the approach you use in the example we avoid them, why? because we will use a single file for index and another for contact, only if we used a preproccessor.