It's all a question of how you perceive "value".  If you only care about the fiat value of your funds, then sure, reserves are fine.  If, however, you care more about the value in terms of utility provided by Bitcoin, that value cannot be replaced if the exchange is hacked and loses all their BTC.  But then, if you cared about the utility value, you probably wouldn't be storing your coins in an exchange in the first place.  People who keep their funds in exchanges are like a different breed.  I don't understand them and they definitely don't care about the same things I do.