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.
10g since 4/18/2012; 10W x2 CFL; Fluval U2. THE FRATERNITY OF DIRT #152