Responsive grid system is a Fluid (responsive, adaptive) layout is today a default requirement when developing a website and, whether we want it or not, we somehow re-orient our thinking towards a tablet-like approach in working with website layouts.
So, the big deal is not making, let say, two columns with gaps between them and making them fluid (putting their width in percentage). The tricky part is when your designer drives you crazy with the fact that the gap is changing along with the browser size. In that situation, it is useless to explain that 2% of 900px is way different than 2% of 600px. In a situation like this, you need something that will calculate widths for you.