Fine if you are a one-person team working on a small project, but when you have a team of several people doing uncoordinated frontend and backend changes simultaneously, chaos can often ensue. Also a lot of 'urgently needed' changes don't survive the few days until the start of the new sprint, so you would actually waste a lot of time chasing wild geese if you are just jumping straight in every time. Two weeks maximum wait (with a week or less very likely) seems like a reasonable tradeoff to me.