QR codes are ugly. They're intrusive. Most designers hate them because there's no way to make them look any less like the brick-full-of-blocks they are, especially when they've been slapped next to a great-looking retail marketing image. That's why the idea of leaving out the QR code entirely and just getting a mobile phone to react to the image itself is so appealing. It looks so much better that it's easy to forget why it's a bad idea: That ugly, intrusive QR code screams "Point your camera at me!" An ordinary image doesn't.
As a result, if potential customers know what they're supposed to do with a QR code, they can easily do it. But how are they supposed to know that there's any special significance to the image in an ad or porter or brochure?