Yes - because right now there's a high volatility on individual markets and some of the listed markets only have a very low volume.
This will change. You see arbitrage on all types of financial markets and Bitcoins are no exception.
There will be differences in pricing no matter what. Look at Ebay,they are still trading at around $25 dollars there , which is amazing considering what just happened at MTGOX.. That point is irrelevant.
The point at hand is: Will scheduled trading give investors a clearer, and more predictable future for their investments.
The answer at this point in time is :Yes: