If you're going to use manual iris, then of course the backlight function is useless; but if for whatever reason you need to use auto-iris, that's when backlight may become more useful. You could also accomplish it without using backlight, by using the A.Iris scene file setting.
Speaking of a.iris. I use it to get close the usually fine tune with the wfm and manual iris. I find it comes in a bit hot so I have it set to -2. Could be hot since cine-d/v like the exposure down a tad.
Sometimes if it is literally a run and gun I'll use auto iris till I can get my hand to the manual control. Hard to do sometimes when you are actually running.
Was much easier to run with a shoulder mounted ENG camera. I could handle zoom, focus and iris with my right hand while pushing people out of the way with my left. lol