This is a great reason I didn't think of, but I suppose they can have a setting to dim or something similar to existing.
Always on is definitely not needed but I know it can annoy and frustrate people if they can't just turn the wrist and see it. Some get angry that they need to spend another half second touching the screen but I also understand. We look at the screen for less than an hour out of 24 so the need is an old way of thinking. Probably impossible to make raise your wrist 100% reliable. Fails for me once or twice a week.
Other brands can have always on but from the ones I have seen in person, the screen and quality is just not there. You sacrifice something for always on and I don't want to sacrifice the quality of the screen.