I used the Take-A-Look mirror for years. Actually, I combined parts from their standard and compact versions to get what I wanted. I clipped it to my glasses. Loved how I had zero vibration in the mirror. I tried handlebar mirrors, but were all but useless for me due to vibration.
However, for the last several months, I've been using theTriEye glasses with integrated mirror and these are working out very well for me. My field of view is a bit less as compared to the TAL mirror. But that really is just a difference rather than a problem. Same with the fact that the mirror doesn't have my sunglasses between it and my eye. The image is brighter and clearer than using the TAL mirror. I thought having a brighter area in my field of view might be an issue, but I'm completely used to it now and it hasn't been an issue.
Admittedly, the primary reason I tried the TriEye glasses, was the high dork factor of the TAL mirror, but also, having the mirror built into the glasses, and not protruding from them, means I can put the glasses in their case, or anywhere else and not bend or move the mirror.
FYI, the TriEye are well made sport glasses with relative thick lens and frame materials.
Just like driving a car, it's helpful to know what is coming up behind you. Sure, not so much for someone who is going to simply run into you, but rather to know what is behind me if I need to change lanes or similar. Or if there is a hazard in the road and I want to move around it putting me closer or into the traffic lane. I can look in the mirror and see that there is or isn't something there to worry about. If there is, I can choose to slow down and let the car pass before moving over, I can simply choose to stop, or take some other action as needed. But it's nice to know that when no one is coming up behind me, I can simply move over and not change my pace.
I also will often move a bit further right just as a car comes to give myself a little extra room. I too gave a Garmin Varia and this just adds another bit of information concerning what is happening behind me.
I recently rode the Iron Horse Classic Quarter Horse event in Durango, CO. My first time riding around literally hundreds of people at the start and always several around at all times. It was nice to be able to glance in the mirror when I needed to move right. Half the time, I would see someone, and have to adjust accordingly. I can't imagine doing it without a mirror.