I'd think that Duchy/Duke might not necessarily like Trading Post. At first it's good because you're turning dead cards (Estates) into money. But then, Trading Post condenses your existing Copper into Silvers, which is bad for consistently getting $5 while buying lots of Duchies.
So as DG said, if you're under attack, the "dead card" phase is a lot longer, so it works out better. It could also work if you play Trading Post a bit weirdly and don't try to trash all your Coppers, but don't quote me on that.
Edit: If there's +Buy and you're picking up Coppers on the side, then you shouldn't worry about overtrashing copper