Sidearms are probably the most under-used piece of kit in the "iron" category. One of my favorite "tricks" is to use them on truss for cable management, such as when multi needs to turn a 90° corner. No more picking cables up off the floor or tying each cable independently. Also keeps the cable from "running" where it drops off the truss to the dimmers, until you can strain relief with shackle and
spanset.
By all means, yes, use sidearms on vertical hanging positions. Those who don't are either too lazy to remove the
instrument's clamp, or just don't know any better. Now if you
don't have sidearms, I've found hanging a fixture (or even a mover) with
yoke either US or DS makes for a speedier, easier focus. Pan becomes tilt and tilt becomes pan, but on a conventional it doesn't matter (unless it's a radial!) and all moving light consoles allow the user to "swap pan and tilt."