At the heart of Agile is flexibility. This is designed into sprints that are intended to account for changes rolled into subsequent sprints.
But, think of an overall backlog. A high level estimation will yield a given number of sprints. This structure actually is not very flexible at all. Unless, of course, each sprint has time allocated for reviewing, testing, and bug fixing. This is a slippery slope; a more substantial change can really throw off a sprint. So, how do you address the issue of quality?