The interest model as a percentage of trading fees will probably become the de-facto standard since individuals need to somehow be compensated to take on the risk of having coins on an exchange in contrast to just having them sit in their own cold wallet and it needs to be done in a financially sustainable way.
This is ridiculous. The only reason to have coins on an exchange is to engage in trading. There is no comparison to be made to keeping coins in cold storage.
The questions really are, what is the incentive for the exchange to share its fees with "depositors," and does this actually increase the risk of keeping your coins there.
Plus, it will be interesting to see if this type of innovative model will eventually get applied to more traditional financial firms.
Deposits with traditional brokers are insured by the SIPC in the U.S.