I don't think that clients use mystery shopping as a marketing vehicle per se. However, it definitely is advantageous to them to expose more consumers to their businesses. That free advertising can pay off for them long term if a shopper becomes a loyal patron of that business as well as offers recommendations to others via word-of-mouth.
Also, sometimes clients have shown themselves to be overly zealous in taking advantage of a mystery shopping situation by offering insufficient reiumbursement and pay to cover the basic costs of completing an assignment. This can border on being disrespectful of the effort involved in conducting a shop in a worthwhile manner at times. It would also be easy to see why some could view the inequitable reimbursement as a thinly veiled discount or coupon on the products and services a client offers, yet the client still receives a professional evaluation of their operations.
The reasons for taking on reiumbursement only assignments have been discussed at length on this forum, and they vary widely. In the end, everyone decides for themselves what is of value to them and their individual MS enterprises.
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"Between stimulus and response, there is a space.
In that space is our power to choose our response.
In our response lies our growth and our freedom."
~Viktor Frankl