I don't stem because I'm far too lazy to pick up a foot. People stem because they don't want to release the old turn until they are sure they've got the new one started, so they hang on to the old turn with one foot while they start the new turn (and change the edge) with the other foot. I'd rather just flatten my skis by any of several methods and let the tips move down the hill. Put I have to let go of the old turn and allow downhill movement, which feels risky to a lot of people.
That's a close description and definitely one of the reasons. You don't have to pickup a foot to stem though. The 1-2 move we see a lot can happen simply because they try to go to the BTE for the new outside before the inside one has AT LEAST been flattened, if not tipped more onto the LTE. Throw in a dash of rotary and you will have a stem.