That may be, but for glasses at least a key measurement is pupillary distance, and it is like pulling teeth to get them to give you this. At my most recent encounter with an optometrist - where I had already selected and was in the process of purchasing from them glasses that cost me hundreds of dollars after my insurance discount - I had to ask them no less then four times to write down my PD so I could buy a few pairs of beaters online. They kept "forgetting."

I presume the PD doesn't change after you are an adult so you could reuse it in the future.

My real concern is if I encounter some problems, if I got the exam and glasses from one place, I can insist they fix it for me instead of arguing who is at fault.