I spend time on a number of other forums which represent the newer vehicles in our family. It's appalling to me how many owners post that they automatically participate in the full 'service' schedule recommended by their new car dealer. They pay through the nose for things such as 'checking the computers' and 'cleaning battery terminals'. I've posted the same thing that you wrote above, but I doubt that it ever makes an impression on anyone. People will continue to do what they do.
Thanks to you TR for sharing your professional perspective on selecting a shop, and I'd have no problem being one of your customers! Being a DIYer, I don't need a shop, but here's what I would do if I did want to select one to do my work. I'd chose from a list of every shop in the area which does free estimates, or charges a very nominal fee ($10-20). That would eliminate all of the stealerships, who routinely charge $100 up front, regardless of the circumstances. Not that I'd ever want to go to one of them anyway.
I'd bring them a perfectly running vehicle, and tell them that it often skips and shudders. I'd also tell them there were OEM plugs installed 10K prior. Yes, this requires a bit of dishonesty, but this is after all the auto industry, right? I'd insist on no work done prior to their estimate, and then listen to what each one has to say. Naturally, I'd be looking for the guy(s) who tells me they can't find anything wrong, and the vehicle seems to be fine. A shop suggesting any work (including further billed diagnosis) gets voted off the island. This will find (if it exists), that rare combination of competence and honesty in one easy step.