Only if you're trading more than $1,000 (USD) worth will your commission be more than $7. I haven't personally traded on the stock market, but I thought $7 is what e-trade and whatnot tend to charge...
If you're dealing with more than $1,000 of bitcoins, then good for you and yes, it's probably worth it to try to find another way of obtaining that many. However, at that point it's probably worth it to have mtgox do the work of finding that many for you to buy.
Well I thought bitcoins should remain competitive against paper money. Why compare purchase of one currency with another one to shares purchase? Stock market commisions were always high. On the other hand if you want to buy Euro with USD you pay almost no commision at all on Forex market. Since bitcoins are alternative currency commision should not be same as on stock market.