Arguably over the course of many years this has been discussed, how much small miners actually matter. When it comes down to it 'dust' hashrate does almost nothing and will almost never receive a block (borderline nill), however they eat profits off of larger miners. Mainly larger rigs with more hashing power. A pool could actually graph this and figure out if the lower 10% of the contributors are actually contributing 10% of the findings. My guess is that's a 'no'.

I know it's not a popular answer, but extremely small miners don't actually do anything useful for a pool. I've been on other pools where they 'weight' your earnings according to some made up system which gives larger rewards to larger miners. I'm still not certain if something like this is fair from a 'doing the right thing' standpoint, but it is from a economical standpoint. You actually get rewarded for work that actually yields rewards rather then your 'contributions' which never amount to anything.

There are tons of different systems, that was popular for Burst mining. Not sure if it's still done that way today. There were a few different types of pools for mining that.