And for a reputable site, I don't think they will have to go through cheating the balance for the sake of scamming users.
That's what I'm talking about. Imagine that you deposit 1 BTC to some casino or betting website and one day you want to withdraw and it says that you can withdraw only 0.5 BTC. What can you do about it? You can never know where did 0.5 BTC go because internal transactions are not public. If every user would have a wallet and if every transaction would be recorded on a blockchain, everything would be publicly visible.