For those many Nikon shooters who use BBAF (Back Button Autofocus) this switch on most of the Nikon lenses is meaningless. For others it might be usefull, as on the smaller Nikon Bodys you have to disable AF in the Menu, not having an AF Switch on the Camera (like D750). But wait, even then you can make a manuall override. Half press the shutter, focus manually (override the AF) and you are done. But for the next picture you have to do this again.
Every button or switch can be useful or can work against you, it depends. What happened to me was that my 35 mm f1,8 AF-s lens was set accidently to M on this switch. Taking pictures in a quite dark church of people celebrating with f2,2 at 1/80 sec and 3200 ISO does not leave much room for "better" f stops. I was concentrating on the people, the lens was in M mode and at about infinity distance, the people were at 4-5 m distance. That meant that I was not able to see in the viewfinder that the lens was set to the wrong distance (this shocked me more afterwards than the fact that I made a technical mistake). Certainly the Af confirmation beep was set to "off" in the church. Yes the AF circle/triangle in the Viewfinder should be a warning, but it is tiny, very easy to forget about it in the heat of the action.
So my solution is: Tape this damned switch and forget it.
And if you have not tried BBAF - do it, it is very usefull.
