Interesting thought, but here is my view, for example the bank is handling 100 crores deposited by their customers; that money from the bank's perspective is something they can not touch at all, they would not be able to do anything with it, while it would have earned an interest of 7.2 crores (@3.5% interest compounded quarterly) in just a week's time with an FD!
Now that is a lot of money to be earned by the bank.
From a customer's perspective, I am putting up my savings into an account, my hard earned money... Why would I pay someone from a bank to keep it safe, and not get anything in return. Bank's like SBI, Central Bank, HDFC bank, etc charge a few rupees anyway as part of annual maintenance charges if a certain amount is not available per month / per quarter.
It might look like a great idea right now, but not really sure about it sustaining for the long run.