> More expensive, too, but sometimes security should trump expense.
All security is a trade-off between 'enough security' and expense. There is no such thing as 'absolute security', if dedicated lines are not measurably more secure than the public internet then choosing to route your traffic via the net rather than via dedicated lines is the right decision.
Banks are in general not 'stupid' when it comes to what they do with their data, they try to do their best within the triangle of security, expenses and technical demands.
If you think using 'public communication methods' for the transport of core financial information is stupid then I guess you are also against banks that are connected to the internet using websites, against banks that use 'swiftnet' (SWIFT is a provider of a secure network used to transfer financial information between banks, which - surprise - has been compromised in the past by the NSA and where payments in transit between two other countries has been seized by the USA because those payments violated a US policy).
In the end, at some point a bank will have to trust the wires and the parties that maintain those wires. From a banks point of view an operator like SWIFT and the public internet vary in degree as to their security but which one to use will always be a business decision and as noted above even SWIFT is not 100% secure and might in some ways be less secure than the public internet.
I also wonder how you propose banks communicate with their customers and with their branch offices.
Here is a nice page on the connectivity options a SWIFT partner gives to its customers all the way from DSL to dedicated lines:
http://www.orange-business.com/en/swift-connectivity
And dedicated lines can be tapped too, so in the end the encryption matters more than the physical connection.