I rethought the PPLNS system last night and think I overlooked an important aspect.
Say the pool is in the middle of a block of usual "length". When you leave at this point, you will get not credit at all for that block. But if you join the next day, the pool is in the middle of such a block again, you will get full credit for that one, even though you haven't mined since the beginning of that round.
Thats exactly what Reikoku said a few posts back, I must have overlooked that too, sry

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So this method shouldn't be disadvantageous for non 24/7 miners after all.