The military learned with the Iraqi invasion of Kuwait that mobile collection assets would be necessary to reach, and quickly focus on, new crisis areas. Most of the fixed eavesdropping stations developed during the Cold War were out of place because of their focus on the Soviet Union, and satellites were either overtaxed or couldn't get close enough themselves. But most important, what airplanes can do is stimulate. Whereas a ground station or satellite is a passive listener, aircraft create their own stimuli. The EP-3 flies a track south of Chinese airspace, and radars are turned on, interceptors are scrambled, communications networks are activated.
Every flight is a potential intelligence bonanza.
Flying Safe None of this is to excuse the collision between the Chinese fighter jet the EP-3 that was flying in international airspace. Given speculation that the Bush administration will take a harder line on China, it's not very likely that spy flights will diminish in the future. But there may be a change afoot that will diminish the chances for the such incidents in the future. President Nixon suspended manned flights and the military scrambled to develop an unmanned platform to operate in sensitive areas. Even in those days of the real Cold War, the drones had a fundamental difference from manned flight.