I think this boils down to the basic rule: don't disrespect your audience.

I keep going to meetings where people suggest using QR codes to advertise a product. And I ask the obvious question: when have you ever scanned a QR code and learned about/bought something? No one can ever answer. But there's a vague sense that somewhere, out there, "those people" are using QR codes.

What's funny is no one seems to know who this is. Among the hip, young, techy people at a startup, it is assumed QR codes are for technically-illiterate old people who don't know what a URL is. Among well-dressed, professional adults, it is assumed they are for all the young, hip people with their smartphone addictions.

I assume someone uses QR codes, but I don't know who, and I don't see why they are popping up in so many places.