There are at least a couple of ways in which having more AF points, such as the systems in the D300 and D7000, is advantageous...
First, with a static subject that you wish to place off-centre in the frame, more AF points give you more flexibility to choose the point that is exactly on the part of the subject you with to focus on.
Second, with a subject that you want to track as it moves around the frame, having more points makes it easier for the camera to keep the subject in focus. With something like a D80, it's easy for the subject to "get lost" between the more widely-spaced AF points.
However, there is a slight downside - with a greater number AF points the camera processor has more to do and this can sometimes result in missing focus with a fast-moving subject. With my D700 and D300, I always choose the AF pattern with the fewest active points that I am confident of keeping on my subject - that's usually 9, sometimes 21, rarely all 51.