the idea was drawn up for hitsnorth, who wanted to have a refuge off his main sump to keep LR
and macro in, for extra filtration I believe.
obvious things like the main drain, main return pump, skimmer, heaters, etc, are left out of the diagram for simplicities sake
whether or not hitsnorth is concerned about his micro organisims getting shredded, I don't know
but I would have to agree with fishfreek in that a lot of very small creatures can easily make it through a pump, otherwise the entire tank would be void such things, since they all eventually get would get shredded by powerheads and other pumps