Have you ever felt like a cashier was judging every movement you made from the moment you walked up to the counter? Maybe you didn't swipe your debit card the right way, or you pay in all quarters, or the cashier is extremely attractive and all you can think about is the fact that he/she is going to see that huge zit on your face? The matter of the fact is, a good cashier notices everything.
Think about regular conversations you have when meeting new people. You aren't required to listen to someone when they approach you, and you aren't required to care about them, or watch their body language etc. In a customer to cashier relationship, it is the cashier's job to listen to and satisfy the needs of that customer. Hence, they are required to watch you, listen to you, and care about you. Throughout a transaction, a cashier identifies, interprets, and reacts to customer commentary, requests, and body language. A cashier can also link or compare these things to what the customer buys, how much they buy, when they buy it etc. They can literally serve as ethnographic researchers, and could be utilized by sales and marketing teams everywhere.
One could say that you could just install cameras and watch the interactions from an objective standpoint. However, any good researcher knows that some of the greatest insights have come from subjective notes. You could watch the interactions and try to interpret a customer's body language, but you couldn't physically feel the emotion that was reflected onto the cashier through the customer (or vice versa). It would be impossible to interpret the interaction without the full understanding of the tone of voice used, or any other minor subtlety that affected the interaction WITHOUT taking the cashier's input or notes into consideration. Hence, you could miss key factors that influenced each interaction that only the cashier would understand. How can you say that customer service is the key to consumer loyalty, but not take the time to listen to or collaborate with the people who generate that consumer loyalty?
In conclusion, cashiers could be an essential part of the marketing research process, as companies are ignoring potential key insights that could only be generated from the front-lines of the industry.