I did one a few weeks ago. It was a promo shop and paid a $40 fee, I think. I needed to go to the dentist anyway as mine retired and I hadn't been in over a year.
I used insurance on the visit, but was still reimbursed for the amount that insurance paid. All in all, it was about a $200 shop for me. I didn't know for sure if they would pay for the amount that was submitted to insurance, but I was ready to fight for it if I had to.
I was pleased when the whole amount was paid to me.
I don't know that I would do it again, though. I prefer a traditional dentist as opposed to one of the high-volume dental offices.
As far as why people don't accept them, it's all about the fee. Shops where you have to reveal so much personal data are intrusive. You routinely have to give your name, driver's license, your SSN, etc. when doing a shop like this.
I never consider "reimbursement" shops to be payment. It doesn't matter if I was going to incur the expense or not. My time is worth something and the "client" has a profit margin built into the prices they charge. I'm smart enough to know that the "reimbursement" costs the client pennies on the dollar.
MSC's all get pretty arrogant when they negotiate and bid against other MSCs for clients. The loser every single time is the shopper. Every. Single. Time. If you want your shoppers to be responsive, pay them. And negotiate those fees when you're negotiating with your clients.