Advantage: We would slow down the idea of time, and stop rushing through curriculum in order to get to the end of a linear line. It would make school less about the curriculum, and more about the personal growth of the child.
The Waldorf Schools are based on this principle to my understanding. Teachers start with a class in Kindergarten and follow them through to Grade 8. Would be interesting to observe and get the opinions of one of the teachers in this type of school?