Only drawback? My bank asks me to remove them while banking (for security reasons), until I tell them they ARE prescription and I need them to sign things!

Some doctors and opticians prefer to avoid AR coatings on tinted or polarized lenses because the residual color inherent to AR coatings is rather visible on dark colored lenses. Smudges and dust are more apparent and the perception is that the lenses are always dirty. Many will order a “backside only” coating to avoid these problems and some AR manufacturers brand coatings designed specifically for sun wear.

Another reason that some eye care professionals might not pair dark lenses with AR coatings (particularly polarized) is that sun lenses tend to reduce light transmission and AR coatings are intended to improve light transmission through a lens. It would seem that these options are contradictory, but there is a compelling reason to apply an AR to polarized products. Polarized lenses reduce glare but still allow light transmission so you can see. An ideal situation would be to allow as much undisturbed light in as possible while diminishing glare. AR coatings also reduce reflections from the back surface of the lens that bounce back into your eye, creating a disturbing image of your own eyeball and lashes or some other intrusion from the light behind you. This is why I am a fan of backside only AR on sunglasses. The cost is the same as coating both sides, but it is worth it. (It is actually more work for a lab to do a backside only coating than a front & back.)

Ophthalmic polarized lenses are designed to address the glare from horizontal surfaces. Altering the axis or orientation of a polarized lens used for these purposes can be disconcerting to a wearer. A misaligned axis on polarized lenses is one of the primary reasons for patient rejects. Although rotating the lens to peer into a reflective glass window is appealing for window shoppers and useful on a camera lens, it is not the best use for polarized lenses in eyeglasses. Most disturbing glare is due to scattered light bounced off of horizontal surfaces, regardless of the direction from which it originally emanates. A rotational ophthalmic lens, in my opinion, would have limited utility and potentially disturbing visual effects.

Polarization is not achieved through tinting or coating a lens. There is actually a wafer within the lens containing the polarized filter. Lenses can be laminated or created by inserting the filter in an in-mold process whereby the wafer is sealed in the plastic during the curing process. Glass lenses are laminated, but plastic are made either way.

Overlaying two polarized lenses at 90° angles to one another will, as you suggest, block out all light. If that is one’s goal, I think a blindfold should suffice! On the other hand, I would be wary about blocking light transmission using this method. The objective of polarized lenses is not to inhibit vision, but to enhance it by reducing glare with as minimal an affect as possible on light transmission. This is one reason polarized lenses are typically a bit lighter than tinted sunglasses. This is also another reason that tinted sunglass lenses are not as safe as polarized. There is a tendency for people to ask for very dark, almost opaque, sunglasses to address bright, sunny conditions. All that they are doing is interfering with their vision and not helping address the actual source of discomfort, which is glare.

You’ve also brought up internal reflection. This is an issue affecting any medium that refracts light (especially diamonds where it is desirable) and it is the result of several factors related to index of refraction and critical angles. AR coatings help with this and it is wise to avoid things like polished edges. This is a bigger topic and I’ve already responded with far more than I suppose you expected, so I won’t elaborate on optical physics and spare you more of my penchant for verbosity.

Sadly, besides cost, weight, and user training, there would still be complex issues of internal reflection to contend with. But I’d love to see optical engineers work on the issue.