1.) If they are mounted to the frame, the finish comes off the frame. (Let alone a semi-rimless lens.)
2.) You have to take off the glasses to mount the clip-on, which is no more convenient than changing to a sunglass.
3.) Magnetic clips which are designed to reduce frame wear-and-tear and increase convenience of mounting (one-handed without frame removal) are likely to fall off, and won't adhere properly when frames are misaligned.
4.) Plastic pressure-mount-type clips (Silhouette, e.g.) are difficult to manipulate, period.
5.) Of course, they are heavier and less optimal, optically.
Either they are secure, but too hard to mount and ruin the frame, or are easy to use, but are insecure and touchy.
Advantages of clip-on's:
1.) Short-term financial advantage for the patient, until their frame is ruined.
2.) It's easier to walk around with a clip-on in your pocket than your sunglasses
We are going to push sunglasses over clip-ons. I hope this clip-on stuff dies a slow death.
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