There are as well many other exchanges with arbitrage opportunities. What about them ?
You're right, as I'm currently conducting arbitrage between BTCChina and other major Chinese exchanges with BTCE, Bitstamp and Mtgox since the spreads have been far wider. An example of which would be that massive rally earlier this week, where at one stage there was a 58% arbitrage spread. By taking it then would pretty much guarantee a solid ROI, which is way better than a gamble on it's continuous astronomical rise.
Problem is, we run out of coins to do arbitrage usually when spreads are at their best. That's why I'm seeking out partners that have money in other exchanges to convert them to coins at the right time.
I believe a mixture of speculation and arbitrage yields the best results, as both hold two very distinct risk profiles that complement each other during different stages of the cycle.
Drop me a PM or add me on skype (id is "aspiringentrepreneur") if you're keen to discuss how to work together.
Cheers,
Sam