The addition of Litecoin or any other number of altcoins to the exchanges won't make any difference to combating arbitrage. That's because the arbitrage arises from friction caused by moving fiat around and not bitcoins.

For instance you can buy a bitcoin on Bitstamp for $101 and sell it on Gox $110 at the moment. A perfect arbitrage opportunity you might think but withdrawing USD from Gox is difficult and that's the reason behind the price gap. Adding altcoins into the mix won't change that.