SMS is the most expensive data service on the planet. It costs more per byte to send SMS then it does to send data to Jupiter (factoring in the space ship you had to launch there to receive it).
WhatsApp's entire business model is based on the fact that SMS is just stupidly expensive.
This generally works out well until people figure out creati e ways to consume levels of service beyond the effective limit -- then the service provider is oversold, and it's either underdeliver or invest in living up to new demand (probably financed with a price increase).
>>> (2<<30)*8/(160*7)
15339168
So there are definitely places where SMS is cheaper than data access. In the US it's inverted purely due to the business model choice of the carriers.
Try to push high volumes through it, and expect to see SMS's delayed - SMS can be delayed for hours at the whim of the operator - or simply dropped, at the whim of the operator. Or simply rate limited.
For all of those countries where maybe the best speed for internet is EDGE or GPRS, but where SMS are cheap, if not unlimited, this service is huge. I don't think the target is rich western world with LTE.