A trader should keep enough reserve to cover his trades and that's it.
Using MTGOX as a bank account is the most retarded thing I've ever heard.
a) you are losing money. very minute you have fiat sitting in MTGOX you are losing money.
b) you risk losing it all, it's a considerable risk leaving large amounts of FIAT at MTGOX. If their accounts get closed tomorrow will you see any of it back? who knows.
Well, if you trade a consistent volume (lets say 5 figures of fiat and 4 figures of BTC) you will need that amounts, which may be very big for you, on the exchange. That's the reality if you are trading more than just play money.