Um. Blockchain.info is not an exchange, and has no reserves (or BTC) for that matter.
/ignore rest of post.
The above is correct. In regards to BTC-E.com, they don't really need to do anything until people start having major problems with withdrawals. The have a large enough user base, and have operated very efficiently for a long period of time. If you don't leave all your coins on the exchange (which you shouldn't be doing anyway), you are only exposed with the coins that you are day trading.