I have never been sold on the concept.
scroll down to #13 here;
http://www.reefkeeping.com/issues/2003-12/eb/index.php
Borneman makes the points more clear than I could. The main points are how relatively small areas are expected to effect large amounts of water and how rock might be better for pod production, if that is desired.
I do keep a 'fuge' but it's more a place for excess rock than any expected nutrient export. And my skimmer is kept there rather than in my sump. It's really just two sumps.
I have always been nagged by the feeling that a properly set up (and 'aged') main tank should not need the extra export, particularly if it has a skimmer, and if more export is needed a simple water change schedule could do the same thing.
nalbar