I know this question has been raised many times, but I feel I have to raise this again.
These 'standard' breakpoints have run their course. We need the ability to design to breakpoints that are project-specific. Currently in Webflow we can't design for smart-watches, TV screens or the vast majority of user touchpoints that fall outside the 'standard' breakpoints included in the UI. I also currently design responsive ads for the out-of-home advertising industry, and these screens almost always fall outside of the current breakpoints. They are so limiting, not to mention the fact they go against the entire philosophy of responsive design..
Adding the custom breakpoints feature will greatly increase the amount of projects I can potentially use Webflow for. Until then, I'll continue to code by hand.