That is completely true,
I guess what I meant to say was:
If you have a long long round like yesterday and you would have stopped mining at hour 12, you would have gotten NO payout at all, whilst when you would have started at hour 12, you would have been paid exactly the same as people that had been mining the entire time (or am I wrong in assuming that? - I had a disconnect for 40 mins during that round because I was directing my miners to the website instead of api.xxx.xxx and my estimated reward dropped to zero, in about 30 mins after I had reconnected my estimated reward was back to normal).
So it makes no sense to drop out of a long round, since you will have mined all that time for nothing, but mining 12 hours somewhere else and then mining here would have enhanced your reward.
But that is only theoretical, since you can never know how long a round will last
And luck evens out in infinity anyway, so keep hashing away at Slush's pool !