Benefits
1 - The helmet mirror extends my Situational Awareness as far as I can see, fore and aft.
2 - You can ride the entire width of a lane, then when a car appears behind you can move to the right. Drivers seem to appreciate that where I live.
3 - Looks dorky? Have you SEEN how road cyclists dress?

I have been using the EVT Safe Zone Bicycle Helmet Mirror from Amazon for many months now. I really like the product, tho it takes some getting used to .... but it is stable and BIG for a helmet mirror.

https://cimg2.ibsrv.net/gimg/bikefor...a4b8b1722f.jpg

Another 70+ guy here. I never felt comfortible with glasses/helmet mounted mirrors. I use the Mirrcycle model that's component-specific and locks into the brake hood. Yes, there is some vibration, but as others have said, I just need to know what's behind me since I can't always depend on my hearing.
To all you young folks who can't understand the need, just wait!

The most effective mirror that I've tried, and use now, is mounted on my sunglasses. Night-time driving glasses actually. lost my shades a while back.

I construct them from craft store mirrors, a paperclip and JB Weld. Drill 2 holes in the frame and wrap it through, close to the lens for a wide frame of view. It's just more adjustable and holds up better than anything I've bought.

I wear ear buds that amplify ambient noise and ride on New Mexico's roads - I hear when cars are coming. I can imagine it would help to know when they're approaching if you couldn't get that info otherwise.

I guess my overall approach to mirrors is that I'm aversive because I consider it too little info, not too much.

At this time, a lot of my riding is on county and state highways, preferentially ones that have a bike lane. One of my nightmare scenarios is a driver obliviously cruising along with two wheels solidly in the bike lane.

A bar end mirror (round; 68 mm) gives me at least some ability to see what's going on back there.

It's just one tool. I can (and do) still use my ears and turn my head, too. Oh, and look in front of me.

I check out every car that comes up behind me, just before the pass me, and make sure their line is OK. I've only had to bail out to the shoulder a couple of times, one time was when a big rig buzzed me at 60 mph.

I do the same, usually bailing several times per year. Here in rural Maine, it's logging trucks and big dump trucks pulling equipment on trailers that give me the willies. I sometimes bail without even seeing them if I hear a big diesel behind me and I'm approaching a blind hilltop or sharp bend in the road. I live or die by my hood mounted Blackburn mirrors.

We use our senses like hearing as well as eyesight to let us analyze danger situations. As long as we use our brains to focus and act on them we can continue to ride.
I am always cautious when I hear any large trucks. The driver may make room for you but they often forget about the lag in the trailer to move out. Some bikers have been hit by the rear trailer wheels. Some trailers have added barriers on the belly of the trailers to help with this.
The road bike with 700x23c tires is not so great when switching on a soft shoulder. Bailing is a last resort.
I am safer on my hybrid with the more erect seating making my glasses mounted mirror give me a better view and 700x38c tires makes a quick evasive move to the gravel on the shoulders easier and more likely.

ANYTHING that enhances you safety is a good thing!!!!!!!!!!!