Designer Chris Armstrong writing for A List Apart:
When we construct a grid, we’re creating layout boundaries: known relationships and constraints that define an environment wherein an appropriate solution can occur. But when we construct an infinite grid, we’re not just setting the boundaries for a layout, but a layout system, with too many variables for us to nail everything down. If we define the important relationships, the blanks will fill in themselves.
Really interesting read, and something that I heard several times in the UI17 conference I attended last week. In short, don’t design break points and your site’s responsiveness based on current mobile constraints, instead design based on site content.