"Safe" is subjective here. When an exchange has the ability to keep you from spending the money in your wallet "as if they owned it," then you trust that exchange just as much as if you sent those coins to them - at least for the time period specified.
Also, I don't see how the exchanges benefit from this system. Why would they spend the effort marking and keeping track of your coins only so that you could roll back a transaction whenever you felt like it? How mad would you be if you made a transaction on an exchange for bitcoins, the price went through the roof, and the next day the exchange told you that the deal was cancelled by the original owner?
The escrow scheme I mentioned is just hypothetical.