I mean aiming vertically in addition to aiming horizontally does present a new layer of skill to the game. The use of a new axis of movement inherently presents more opportunity for skill expression than if that axis of movement comes with incredibly detrimental penalties to the point where it's not useful to use that new axis of movement at all.

Being able to flick onto a target that's diagonally away from your crosshair takes more skill than flicking to a target that's only to the left or right of your crosshair. With a controller the amount of skill required to aim on both axis is excemplified because one of the most common ways to aim with a controller (at least with snipers) is to drag your crosshair across the target, wait for the aim assist to get "sticky" and then simply time the trigger pull, you're changing the "aiming" portion into more of a "timing" game. With a target that's moving on both the x and y axis you need to now get your drag angle correct and account for the trajectory of the other person falling.

The freedom to use another axis of movement objectively adds a new layer of skill to the combat.