Controlled feeding may be better when loading upside down, but that was not the original reason. Mauser actually started with what amounted to a push feed, but found that (then-new) pointed bullets created a problem more serious that a cartridge falling out. In a push feed rifle, if a round is chambered but the bolt not fully locked, the round will not be extracted when the bolt is operated again. Instead, the bullet of the next round will end up being driven into the base of the chambered round. If the bullet has a full metal jacket, is pointed, and happens to drive into the primer of the chambered round, very interesting things happen shortly thereafter.
Controlled round feeding (or more precisely controlled round extraction) prevents that kind of nasty surprise. Modern push feed rifles are generally sporters using soft point bullets, the shooter very rarely rapid fires, and the feed lips are controlled so that the cartridge point is guided away from the center of the chamber. But gunsmiths who alter those actions for cartridges with a differently shaped case should be aware of the last point and watch for a potential problem.
Jim