Phoenix is certainly a different market than where I'm at, but, around here there is no way a "dyno only" shop could make it. The shops that do provide chassis dyno service are regular automotive shops that also provide additional services, which include the possibility of dyno time, or are emission testing facilities that are working to minimize emissions. But I live in an area of only half a million people. Phoenix is what, 2.5 mill?
I've been to three different shops with dynos. All three times they were part of a club activity so they weren't really "tuning" sessions. Everyone there that day got three-five pulls just to see what we were putting down. Speaking to the operators, they do offer regular tuning sessions that are typically $100 an hour. They all allow the vehicle owner to assist in the tuning process, but have said that typically customers want to maximize their time on the dyno, therefore they end up do much more tuning than the customers. This is because they do enough cars that they can more quickly interpret the data and make adjustments than a customer can.
Testing on a dyno isn't any more stressful than stomping on the gas while driving a car. This is another part of the reason the shops want to control the pull. They will monitor the output and stop the pull shortly after the engine peaks in output whereas many owners may continue to try and push rpm beyond a necessary point. Really, if power is dropping at 4500 rpm, why continue the pull to 6000. Suprisingly, many cars do not need the rpm, nor produce the power, many owners think they do.