There are significant advantages and disadvantages to investing in this defense contractor.The bull-and-bear debate over Lockheed Martin (LMT +0.39%) and defense stocks in general rages on and is unlikely to be resolved in the near term. While the bulls emphasize the conducive environment for defense stocks, the bears are asking serious questions about the long-term margin profile of these businesses. It's a fascinating debate that every investor should consider before investing.The Bulls' case for Lockheed ...