That then adds yet more risk - a third party who could disappear with the bitcoins overnight and there wouldn't be anything anyone could do about it.
Also, people who buy bitcoins often need them quickly. Planning finances one month ahead won't work for many users (i.e. silk road). Also, what would happen if the value of the bitcoins drops by a few hundred percent during that month?
There are far safer ways to purchase bitcoins than paypal. None are risk free though.
There isn't a symmetric trust scheme between paypal users and exchanges. There are many reputable services out there who hold Bitcoin balances. I hold coins flexcoin for instance and trust them for that many coins, if not more. A lot of people keep some coins in exchanges. The idea is, your balance shall not exceed your trust. The service needs to build reputation, or must be built by reputable members for the users to trust. Since the service will be holding the coins for a short period, it's even less risky. It can be a registered company too, if there is a trust to be gained by regulations.
My original idea was that buyers buy the coins at the market price at the time of purchase and the exchange keeps them for a certain time. Though, as you said, there is still a risk for the exchange, since the price may drop and buyer may ask for a chargeback. A much lowered risk, but still a risk.
There are not many safer methods to buy with paypal if you don't have any reputation. And there isn't a straightforward way to buy at all.