[mortenlarsen]

There are other concerns than just the amount of KB transfered. When connectivity is poor SMS is much more likely to work than data (2G/3G/4G). AFAIK. GCM is not a "true" push service, it just provide an API that makes it seem like one. Battery use is much higher for data, especially when you are in a location with bad connectivity. Some people prefer disabling data when they don't need it, for the reasons above and for other reasons.

[xorcist]

Most people disable their mobile data when roaming, because fees are set to bleed the unfortunate few who forget to dry. If you live in a country that is only a few hours drive to neighboring ones, that's something you do quite often. So SMS is not only more likely to work, it is absolutely required.