When we call them nitrate traps, what we really mean is nitrate factories. Nitrates are produced when organic matter is decomposed, first into ammonia, then into nitrite, and finally into nitrate. While nitrate is the least toxic of those compounds, none of them are good to have in the water. All of this work is done by bacteria. The concept behind bio-balls is to have a lot of aerobic bacteria, since they are the ones that most effectively convert ammonia into nitrite, and nitrite into nitrate. So far, so good. The problem is, with the rise of protein skimmers and the Berlin method of filtration, we use live rock to host the bacteria, but also use protein skimmers to remove suspended detrius in the water column before anything is converted into anything. If you have "dead zones" either because there is no flow in that region of the tank, or because you have bio-balls or whatever, they will trap larger detrius, where it is free to decompose, which inevitably produces nitrate. If you cleaned the bioballs religiously (i.e. weekly) then you shouldn't have a problem, but it would also probably prohibit the bacteria from growing on them, negating any benefit as well.