An SMS based service for checking the balance of a Bitcoin address seems much more useful, since people in very poor places are much more likely to have cheap cellphones than devices capable of processing the whole blockchain (and an SDR or so to read the signal).
SMS services aren't secure. Lightweight SPV servers (well within the resource constraints of ultra-low-end smartphones, which will see increased adoption in very poor areas over the next few years) are. And the whole point of outernet is to reach areas that might not even have SMS capability.
