Stance adjust also has no function, though it doesn't make a lot of sense without a gun. Peeking without a gun should definitely be possible, though.

Currently, you can't even switch weapons while moving. It really seems like a relic of past games in the series and not in keeping with the improved control in Arma 3.

Yeah, I just had this problem too when I was trying to set one button to toggle optics and zoom (so I'd always be zoomed in when using optics).