The reason why you have to still eat a very healthy diet even after you get most of the mercury out is that this world is rather polluted and it is good to have extra protection from supplements. Air and water quality can be pretty bad, and don't forget about the ozone not being what it used to be.

I personally take many 'superfoods' such as spirulina, bee pollen, reishi, etc... and would continue taking these even after getting most of the mercury out because industrilization has really contaminated this world.