if it were always true that the price goes down on the weekend due to inability to wire fiat money to an exchange, the smart traders would spend the weekdays wiring their money in preparation to buy only on the weekends when prices are low. This should level out the weekend dips in the long term
in other words if it was always obscenely profitable to do something simple, everyone would be doing it and nothing else, and then it would no longer be profitable
Funnily enough, this appears to be exactly what has happened... the weekend dips disappeared not long after lots of people started talking about how they were getting their USD into the site for the weekend to take advantage of them.