on average its 2blocks/day
last block was:http://blockchain.info/block-index/852772
each has has a certain chance to be correct, so there is no "not effective use"
the stales/orphan/dead shares you see in the client are for the shares chainblock and not the real one
so its just a bit of bad luck, sometimes you find 3-4 sometimes none. On average its the same as a 0% PPS