It's about time that performing one flight - a complete flight from A to B, not just a short pattern! - with an open door in a training aircraft becomes part of the PPL syllabus. To demonstrate to the new pilots that these airplanes don't care the least little bit whether or not the door is open, closed or missing altogehter. There is some noise and some draft, maybe a little unpleasant on a cold day, but this is it. Just keep flying. So many totally unnecessary crashes and hard landings after rushed "emergency returns" could be avoided.