I think two core ideas of sprints are to 1) combat Parkinson's Law by creating an artificial deadline (the team should try to solve things within the sprint) an

I think two core ideas of sprints are to 1) combat Parkinson's Law by creating an artificial deadline (the team should try to solve things within the sprint) and 2) protecting the team from changes (avoiding that requirements change all the time) With that in mind, sprints can work pretty well, and work "spilling over" to another sprint is generally no problem. It starts falling apart if the artificial deadline becomes a forced deadline, or if the protection part is dropped.