SMSing might be good, but there are also a few reasons not to do so. For example, there are many users who don’t really want to enter their phone number anywhere on the Internet (and it is actually a good point, at least you get more privacy).

A few more reasons:

1. There are always third parties (the SMS gateway, the carrier) when you send SMS messages.

2. You cannot receive SMS if there’s no signal or if there’s no cellular module at all (a frequent use case on tablets, iPods, etc) — just the camera, Internet connection, QR code scanning application. :)

3. Sending SMS messages costs you money. In another hand, QR codes are generated for free. Also, it usually takes more effort to set up (and also it can break — relying on third party services is almost always less solid).

4. SMS messages can be slow (and sometimes you don’t receive a message at all). QR codes, in another hand, are very predictable and usually work pretty well (at least if the camera works correctly).