EDIT: Whenever you begin mining (in a PPLNS system), your shares start being counted there. Even if you know N, there is no point whatsoever in pool hopping - this is because no matter when you join, your share won't change. You can see this clearly because N makes the round time constant - instead of random, like with proportional.
A miner could switch from one Prop pool to to another, to another based on block freshness and use PPLNS as a last resort when
nothing is fresh. Knowing N can determine how long a block is considered fresh enough for Prop.