My understanding is that in addition to determining the proper prescription, grinding lenses to fit them is something that many third world countries don't have the infrastructure for. Self-adjustable, 3rd world-friendly spectacles do exist[1], but at that point, does one really need a prescription, or can you just dial in until you can see well?
[1] http://www.trendhunter.com/trends/josh-silver-adaptive-eyewe...