I don't use a clicker, and try to align myself the way Arne describes. I think the main reason compound shooters don't need arm guards is because of the compound bow's higher brace height. When shooting trad recurves with brace heights of 8" or more, I never need an arm guard. Same with r/d longbows with brace heights of 7 1/2" or so. However, with a Hill longbow braced at 6 1/2", I need an arm guard.
Even though I try to align myself as Arne describes, I don't always succeed. I think I'm properly aligned, but my head droops or I don't expand enough, etc, and I don't reach my full draw length. After a shot or two goes low, I begin asking myself "why", and since I tend to make the same mistakes after all these years, I generally can figure out why I'm doing it. However, this is after the fact. If I were highly competitive, I would use a clicker to make sure I was always at full draw before releasing the arrow. I think all Olympic shooters use clickers for that reason. Not all competitive trad shooters use them, and not all trad tournaments even allow them. It would appear that the top trad shooters can consistently come to full draw without a clicker without making the same mistakes I seem to regularly make. Good for them, but if I were going to compete at that level, I would need to use one.