I tend to think that Nitrates are an indication that Nitrification bacteria have been at work. No Nitrates tells me that plants are doing their job. I know we don't usually get zero Nitrates, but that's when nitrates have come in.
In the end, it's hard to offer a conclusive answer. I think threads like this are good to help people develop their own there own theories, and maybe sometime an answer.
Planted 10 gallon tank; 2 x 10W CFL; Fluval U2; MGOCPM/PETCO black sand cap
The Fraternity of Dirt #152
A Silent Cycled Tank