A wide-angle lens is simple to use, as you have amply read here. Easy to focus (“zero to infinity at f/5.6″), and easy to shoot at slow shutter speeds.
So why splash out on an f/2.8 lens instead of, say, a 3.5-5.6 lens, or an f/4 lens?
Precisely because it is so easy to get “sharpness all over”. What if I want blurred backgrounds? Like in this shot?
For that shot I held the iPhone as close at the 16-35mm f/2.8 lens would focus (namely, at 28cm from the camera). I used f/2.8, wide open, and that plus the close proximity to the phone gives me acceptable blur in the background. f/4 or f/5.6 would be a whole lot sharper in the background; in this case, that would be undesirable.