I was envisaging the exchanges paying interest to build their reserves. For example. Bitcoinica is short of USD at the moment.
Currently, lending Bitcoins to a person you met on the internet seems insanely risky, there is no enforceable way to make the receiver repay the funds. Maybe if they signed a contract it would help, or had other assets in escrow as security for the loan.
Lending BTC to a company (eg. an exchange) might be possible for small amounts, as they put their reputation at stake.
Ah, OK. From a business standpoint, there are much better ways of them increasing their reserves than accessing user money and paying interest on it. They could obtain an outside loan, exchange equity for investment, issue debentures, and a shit load of other stuff which wouldn't raise the same kind of issues as pooling uninsured user funds and using them for other purposes.
A huge issue surrounding them keeping user funds on deposit - even if they quarantine those funds - is that those funds are currently uninsured. If those funds are lost, then it's not economically viable for small deposit-holders to sue for compensation even if they had valid legal grounds for doing so. I'm sure that one reason why the exchanges discourage people from using their accounts as online wallets is the potential liability issue if those funds are lost or stolen. More users leaving more money on deposit would only increase that potential legal exposure.
Shortages of particular currencies are more a function of allowing users to use the service as a currency exchanger than they are of acting as a pure exchange.