An economy on the curve can increase demand but its supplies cannot increase causing prices to rise, or inflation. If there is no deficit and the curve shifts to the right then supplies will not increase and the country will no longer be operating on the curve. A deficit must be maintained to insure that the economy grows with its resources. The trick is finding out how large the deficit should be in order to allow for growth without waste. The U.S. s deficit is bad at this point because the U.S. is close to its maximum production capabilities, and deficit money is being wasted.
Defense spending produces little or nothing except in times of war.
Judging by the current status of the United States as the only existing Nuclear Super Power war is not a tangible event in the near or distant future. The way social security is managed creates a huge waste. As managed, social security is money spent to immobilize a large and fairly capable part of the work force. It encourages elderly people not to work by spending deficit money on them. Reducing productivity and increasing the debt at the same time.