Lack of interest?
Sachdev and others now also see other limitations in the M-Pesa model.
"There are many benefits to mobile money," said Neal Estey of Boston University's Center for Finance, Law and Policy. "But it should not be sold as a miracle cure for financial inclusion."
He noted that unlike in the U.S., where you may use your smartphone to access your bank account, mobile phones in the developing world are often used instead of bank accounts. For many that means having no opportunity to enjoy the benefits of formal banking: interest-bearing savings products and building a financial record to show creditworthiness.
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That need not be the case, said Haridas Nair of SAP, the German multinational enterprise software company, which has focused its mobile-money efforts on Asia.
In Bangladesh, where 87 percent of people have no bank account due to poverty and/or limited access to banks, poor and remote Bangladeshis had one thing going for them: Most already had basic cellphones.
Within one year of launching a mobile-banking program, Dutch Bangla Bank saw that withdrawals were becoming less frequent and customers were increasingly using the system as a de facto savings account. So DBBL initiated a program to allow them to gather interest if certain conditions, such as transaction frequency and length of deposit, are met.
"It's possible because a bank is running the service," Nair said. "With M-Pesa it's a telco."
Safaricom has an answer for customers who are hungry to earn interest and build credit: Last November, with the backing of its partner Commercial Bank of Africa, it launched M-Shwari, an interest-bearing savings account that also offers microloans, and it already has more than 1.2 million active users.
Banking on telcos
Still, Nair argued that because the telco runs the show, the bank partner is limited to selling a product to that telco's customers. "In a bank-led model, the banks needs to work with telcos for connectivity, but they are able to reach an audience that is across (numerous) telcos." Nair admitted that M-Shwari does offer something unique: a "semblance of a credit rating" that would be hard to otherwise come by for people with no banking history.
With banks acquiring mobile networks in some places and mobile networks acquiring banks in others, it may continue to be a chicken-and-egg issue.
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"We're agnostic to models," said Khalid Fellahi, head of Western Union Digital, which partnered with M-Pesa in 2008 and now facilitates money transfers between Kenyan subscribers and people in 45 countries and territories. "Telcos provide reach that banks cannot, and it wouldn't be sustainable for them to try," he said. He added that regulators are still catching up to the fact that an M-Pesa user with the equivalent of a couple hundred dollars probably shouldn't face the paperwork and red tape a bank customer does. "It's a stored-value account like PayPal, where you withdraw funds as needed," he said.
Fellahi said Western Union makes it possible for a worker who has gone abroad for employment to avoid the delays and security risk of carrying his earnings back with him if he's willing to part with the $7, for example, it costs to send $100 home (within Kenya's borders M-Pesa's remittance fees are about 40 cents). Fellahi provided no numbers but said the service is "very popular" and continues to grow.
Just where mobile-money systems will be in a decade is hard to say.
It's a good bet M-Pesa will be going strong in Kenya. Even a 10 percent excise duty the Kenyan Treasury imposed on mobile-money transactions in February did not stop them from growing 22 percent over the past year, according to the country's central bank.
Elsewhere, big bets are being placed. Spanish telecom company Telefonica is introducing mobile wallet services in eight countries in Latin America and Europe and aims to expand that to its entire customer base—300 million people in 26 countries—in the next couple years, according to the 2012 Sybase Mobile Commerce Guide.
Fellahi said much will depend on two things: slow-footed banking regulators getting with the program and operators figuring out ways to standardize protocols.
Sachdev argued that while "it's fantastic what M-Pesa has been able to do," the future of mobile banking will sooner or later belong to banks.
—By Matt Twomey, Special to CNBC.com
Follow him on Twitter @Matt_Twomey