many will not even think about the vertical format and end up with a lot of empty space on the sides of the subject that could have been eliminated by simply going with a vertical format. some subjects are obviously suited to a vertical format. but, having said that, a tall subject could also work in a horizontal format. if you wanted to show the whole waterfall, fairly close in, i'd use a vertical format. but maybe you want to show the setting around it as well. so you back up or use a wider angle lens and include some of the rest of the scene in the image. most cameras are more comfortable to hold in the horizontal position. but the opportunities for vertical compositions are just as numerous and shouldn't be overlooked. certain subjects IMO are custom made for this format.
Well put. Clearly you have a broader view on the subject than me. Can "rules" like the thirds and the golden triangle be applied to vertical shots? I am sure they can. True; certain subjects lend them selves to a vert view. Still I am uncomfortable with the whole deal. I have a 2 1/4 square format camera that threw me for a loop for years.