I like the idea if there's also a lending market attached to it so that savings HBD can be the risk free rate and those looking for HBD can also borrow some at various rates set by users so there a risk free rate and a P2P rate!
There's an opportunity cost for locking up for 3 days and if their are various escrows you can lock into to get better returns then it makes the 3 day less of a pain
If there was also a burn fee to break the 3 days to 24 hours that would be cool too!