I'll outline the argument, and let you decide. (I made a lot of generalizations below regarding developing economies/countries, I'm sure I'm wrong in some cases, this is just what I know largely to be true)
>Remember that in many of these countries, most people don't have smart phones, and there's a lot of people who don't even have phones.
Depending on the country you're talking about, smartphone penetration for a lot of Africa is over 20% by population, increasing with increasing wealth or earnings. Cambodia, from your example, has about 50% smartphone penetration and 98% mobile phone penetration.
>Electricity and internet connections are intermittent at best.
Anecdotally, I can report that 3G coverage is ubiquitous in mid-size or larger population centers in Asia and Africa, even in very poor countries. In the countryside, it is hit or miss.
>Good luck convincing Cambodian street hawkers that your magic internet money is better than a good old fashioned Greenback.
I don't think small scale transactions are the use case for developing economy cryptocurrency utilization. Rather, the argument is that households may prefer an alternative to their local currency which allows them to hold liquid or semi-liquid savings. In developed economies we have bank accounts, stocks, bonds, CDs, and many other vehicles allowing for liquid store of value, and we (mostly) trust the government and banking system. In developing economies, lower and middle class households often have only cash vehicles. Some may have bank accounts, but those are subject to governmental whims (see India's disastrous surprise currency change last year) or currency volatility. When possible, many households will hold long-term savings in USD or EUR cash notes, as those are perceived as safe (wealthy households typically do the same, only by having a USD or EUR denominated bank account with a local or, preferably, international bank). Households save up local currency and then change it out for these high denomination USD/EUR cash notes with a bank or money changer. They would not go use $100 bills at the local market for groceries, they would change their currency back to local fiat if needed. Alternatively, they can sometimes transact large purchases with USD or EUR notes for things like computers, cars, property, and livestock.
Essentially, the argument for cryptocurrency as a store of wealth in developing countries is: 1)households would prefer not to hold notes, as they are more easily lost or destroyed than a backup key would be 2)there are fewer transactions to deal with in exchanging currencies 3)remittances are easier and lower cost 4)holding high value notes only works for households of a certain wealth class, cryptocurrencies will allow lower income/wealth households to do what middle-lower income/wealth households are already doing