I've used them for years ... long before they were well known or mandatory. I've probably heard ten thousand times "what's that in yer gun buddy?"
Even though you can see a gun has an open action (on most action types) from a distance using a flag makes it that much easier, and not just for you but for all who are on the line. As an RO I always appreciate the ability to see the status of all guns, pretty much from any point on the line.
There is another reason I like "proper flags". By that I mean Empty Chamber Indicators, not Open Bolt Indicators.
Many years ago only OBI's were required when shooting NRA Highpower matches. That has changed but at that earlier time the NRA caved to a huge outcry of shooters who were convinced that placing a piece of polymer in their chambers would cause all sorts of ills, from rashes to guns blowing up. Okay that was sarcastic, but if you were around matches in those days you know of the belly aching I'm talking about.
On the day of which I'm thinking, a very seasoned High Master shooter was loading gear in his car after the day's match. He pulled the OBI out of his AR pattern service rifle, dropped the bolt and pulled the trigger so he could then case his rifle. The round that was fired hit the roof supports at the 300 yard range, where a number of us were standing. Several of us were hit with wood particles from the impact.
I know what many are already thinking ... yes he violated basic safety rules. He should have visualized the chamber. Yadda Yadda Yadda. (I'm not trying to minimize the safety rules or that he made a bone head blunder ... I'm trying to deflect that thinking because I have another point to make).
Here's the point ...
This guy had been shooing for decades without an accidental discharge. I had shot with him for years. He is a safe shooter, just like you or me. I continued to shoot with him after this mishap. He competed for another ten years or so and never had another problem.
It was just that one time.
One time where his attention slipped for a moment … over a lifetime of shooting.
But ... it only takes one time.
Had he been using an Empty Chamber Indicator this would never have happened because you can't insert the blasted thing unless you have ... yep ... an empty chamber!
So I'm sold on them. Even if it stops one AD that doesn't do anything more than embarrass the shooter and keep a few guys from jumping a few yards in the air from the fright, they are worth using. They are low cost, almost idiot proof and almost fool proof insurance.
Just my take on the issue.