This is not quite true.

• To be paid for the work you did, you have to go for a PPS pool.
• There's effectively no difference whether your new shares pay more than the old ones or the old ones pay less than the new ones.
• Your score (value of shares) is recalculated from time to time in long rounds. The ratio of the old shares to the new ones is kept, but their absolute value is not.

These are facts to be corrected relating the quoted post.

In fact, you are punished for disconnection. You're punished exactly the same way you'd been in PPS in the long run, provided that you disconnect asynchronously, i.e. randomly.
At Slush's, you can either lose, when you disconnect near the round end, or you can gain, when you disconnect at some other time. In the latter case, your payout for the round won't be decreased due to the disconnection, although you would deserve some decrease. And as the time constant c is hopefully still 300 seconds and an average round is several times longer, it's greater chance for you to gain a bit than to lose substantial part of the reward.
Anyway, in the long run that should even out, as is the popular phrase in this thread for some time already... :-)