You are looking only at one side of the medal.
Yes, it takes more time for the fiat to enter MtGox, but once it's there it's trapped!
It's not getting money in that's hard, it's getting it out.
So if BTCs can move out at anytime, but cash must stay on the exchange, why would that only be 'downward pressure'?
Furthermore, the weekend dip is a myth.
Cash stays trapped on the exchange, but isn't available for transactions once the withdrawal process has started. That means that, even if it does take days for the fiat to come out, once you've hit the withdraw button, you can't get it back and use it to buy BTC.
The weekend dip isn't a myth, it's just not a certainty. Again, it depends on the foresight of the traders. During times when most of the money flowing into Gox is "new money" from people who have just become exposed to Bitcoin, the weekend dip is pretty accurate, because as soon as the deposits go through, they want to buy.
In situations like now, when the money sitting in Gox probably belongs mostly to people who have been around Bitcoin for a while, I agree that the dip may or may not happen. Nevertheless, I think the chance of it happening is greater the longer the banks remain closed.