//"A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative." //

Good point.

Another sample and how useful double negatives are, the singer of the Rolling Stones sings,can't get no satisfaction, the 2 negatives make a very strong NO instead of,can't get ANY satisfaction. Best.