That's not really abusing the score system, since doing that you will actually end up with less coins than if you just mined on Slush 100% of the time. When you're pool hopping you want to mine on the proportional pool (or Slush's Pool) at the very start of a round, and then hop away after a certain period of time if a block hasn't been found.
I thought with slush's pool, if the block is long and you only contributed in the first portion of the block and you leave elsewhere, your score/shares becomes useless.
On the side note, what I'm trying to say is why should a person get almost the same payout if they only came and mine for the last 5 hours of the long block, when someone has been contributing for the same whole 17+ hour block will get similar payout also.