In studio, and situations where the photographer can control the people and objects, fully manual flash is desirable. But in events with different lighting in each place that you point the camera, TTL flash gives a much higher percentage of correct answers. In events you better have some slightly dark (or very light) shots that did not make the photo because I was adjusting the flash power all the time.+1. Most of my outdoors work with flash is done with ETTL. Saves a lot of guess work. Adjusting the flash by a stop or two is about the only "manual" aspect I might use with ETTL.