It's been a year since I got my last pair of glasses, so I figured I ought to make the most of my insurance (one pair per year) and get a spare. In contrast with my recent experience with a new dentist, the new eye doctor I tried was great!
I went to Webster Vision Center, and was seen by Dr. Sara Day. The first thing she did was measure my current glasses to get my last prescription. In conversation, I asked if she could then make me a duplicate pair of glasses without an exam , and was surprised when she said it was technically possible, but at best an ethical and legal gray area. We ended up having a really interesting conversation about it.
I always thought of glasses as a product, and the exam was just a measurement. If I had the prescription written down from a year ago, I'd expect to be able to go into a glasses store and get another pair, and this doesn't seem so different from her getting that prescription by testing my current pair. She made the point, though, that the act of her writing a prescription carried with it the same liabilities of any other medical prescription. Just as an MD couldn't prescribe me some pill just because some other MD had done the same, it would be both potentially unethical and potentially illegal to copy out a prescription.
There is also an argument for getting an exam, if only to serve as a general eye-health check-up. It seems somewhat of an unrelated issue, though, since forcing people to care out their health seems tangential to them having glasses.
I was definitely at risk of coming off as argumentative (as at the dentist), but because this doctor was clearly interested in the educator part of her job I left there informed about my specific concerns, and much more! If anyone in the area needs a new glasses place definitely check out Webster Vision Center.
 In my case, it had been a year since my last exam, so the question was mainly academic, as my eyes may have changed.