What AI101 and others are scratching their heads over is that the aircraft they fly will probably fly the SID track very very well and for many aircraft nowadays, the days of wandering to a beacon are long gone. However whilst avionics and nav accuracy have made significant progress, the practice of ATC using headings have been around for donkeys. Locking an aircraft on a heading has and still is a fundamental method of separating aircraft but it dates to a time when the majority of aircraft did fly from VOR to VOR and did at times wander slightly off their track but still be within the tolerances for tracking etc.