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.
This is true, but compared to the rest of the miners on the PPLNS pool, they're not getting rewarded unfairly. If some people are still running proportional payout systems even though it's fucked like it is - that's kind of their issue, and has nothing to do with the safety from hopping PPLNS provides.