Why do you assume it's the banks that need to be kept in check in this case (not talking about all the other shenanigans they may be up to)?
It's cyber-criminals.
Mt. Gox fell because of hackers attacking it.
The only thing keeping your bank balance safe is the IT staff of your bank. Heck, even the goverment would be happy to have a security system that means they don't have to actually insure your money because nobody could steal it.
I'm simply saying, there are potentially useful components to this crypto-currency stuff that somebody (a government/bank) looking to stop printing paper might be interested in.
Otherwise, it is a matter of time before an attack occurs at a bank that involves changing balances in the records, instead of stealing paper or DoS. The mess might be just complicated enough that it will be less about stealing money, more about damaging money. And it will be hard to prove how much was in each account.
I supposed I could just ask the guy on my team who worked reasonably high up at one of those banks...