- works offline (for people who do not have a mobile data flatrate)
- The QR codes could even be printed on visiting cards or similar
- Actually only one direction is verified (if Alice scans Bob’s code, A can be sure that she chats with B. - But Eve could scan B’s code and do as if she was A without B noticing). So, B can’t be able to add A to a verified group.
- B could ask A whether the verification worked and then be sure that he chats with A but DC won’t know
My opinion is that nevertheless, this should be added as an additional feature (like, if a scanned QR code is “too old” or there is no internet connection, show a warning to the user and do a one-direction offline out-of-band verification).