The problem with interest is that you get it in exchange for putting it in the hands of the bank, which can use it to loan to others, etc.
Interest is like payment for accepting a risk. Unfortunately, there are not risks with the FRP bank. 
In addition, only the rich would benefit, and they're already rich enough. I'm not ruling it out, but considering the whole thing is broken in the real world, we don't have a snowball-in-hell's chance of getting it right here
Invisible War ][