It seems you're asking two different questions:

1. What is the importance of demographics?

2. What are the pain points in building facial recognition into a retail transaction?

To answer the first, demographics inform what products are being sold, who gets what promotion, what to start and stop carrying, and more in retail. The degree to which a retail shop pays attention to its clientele is directly related to those decisions and thus to its success.

To answer the second, you will need to deal with the legal ramifications of facial recognition technology, and once you've handled that, you'll need to deal with the range of reactions you'll get from customers who will need to know they're being recognized--not by humans but by robots. I'd say the second issue is the most important one to consider as you shape the technology and how it will be used.