Because with my camera that gives me the result I want. I hate clipping the highs, and my Phantom will clip without dropping the exposure. Each camera is a little different. Your mileage may vary. I have instrumented my approach by evaluating the scene-exposure-gamma result both with the resulting video waveform, as well as visually, on a WELL CALIBRATED monitor. Most computer monitors require calibration to get true results. HD should be calibrated to rec 709. UHD/4K calibration depends on what your monitor is capable of.
By the way, I am using a Phantom 3 4K, so there may be some subtle differences between the 2. The lower end P4K cameras are supposedly identical to the 4K cameras on the 3.
By all means use the Ettr method if it works. Just be aware of the ugly clipping that can happen if you are't careful.