The use of SMS infrastructure for mobile banking makes a lot of sense. Many deployments with this (very popular) mechanism is available in many countries. This type of mobile banking is rather easy to implement and can be accessed on any handset, in any country and with (if not low) at least predictable cost. It is relatively easy to deploy and extend and can be made available to any market within a short timeframe. As a matter of fact, the neighbour's son that recently completed that computer diploma thing, can probably do the integration.Unfortunately, it is also basically not secure, can easily be hacked, the service is not reliable (or predictable) enough (for banking). The user experience is open for interpretation and consumer protection is therefor also difficult. Support costs and training is always a challenge.
Encrypted SMS deployments while significantly more complex solves all of the above problems. Consumers are presented with an easily understood menu on any handset (the same format as all of the other services on the handset). The solution works on any handset is available from anywhere in the world and is almost un-hackable. This type of infrastructure utilise the most advanced encryption technology (piggy-back on inherent GSM security primitives) and is the basis for most of the success stories around (including mPesa, MTN Mobile money, Smart Money and mChck).
One should be careful to distinguish between SMS mobile banking and encrypted SMS mobile banking. The only similarity is the actual carrier technology.