Segregated wallets doesn't work, that would mean every single trade would be a transaction on the block chain. It would absolutely flood the network and result in massive fees. It literally can't be done that way.
So you went from " if the team behind an exchange isn't capable of dealing with the complexity of individual, segregated wallets, you have to wonder whether they're competent enough to run an exchange in the first place" to "that inherent inefficiency"? Which is it, an inherent flaw in BTC, or ineptitude on the part of the exchangers?
