It has not been a problem with the D7000, which had gotten a reputation of being rugged. Heavy lenses are normally carried by the lens so the camera has to only support itself. Stan St Petersburg Russia
I've shot with everything from a D3100 to D4 and never had a problem with the lens mount on the camera. However, the lens mount on my 80-400 did loosen up once. Rather hard to tighten those little screws but it stayed tight.
I believe that Nikon uses for camera bodies and camera lenses polycarbonate which is a high performance polymer with excellent performance characteristics. I think that people are mainly concerned about the "feel" and the "look" of polymers versus metals. Metal parts are heavier, more difficult to machine and more expensive, but not necessarily tougher or more durable.
Heavier lenses were never a problem with my D7000, and haven't proved to be a problem so far with my D600. I tend to use a two-hand grip when hand-holding heavy lenses, with left hand supporting the lens. My *really* heavy lenses are almost always mounted on a tripod attached to their tripod collar, and in that mode the D600 doesn't "know" whether it's hanging off a 70-200mm or a 200-400mm zoom.