Also, Bitfinex almost always has a lower price than the other exchanges so there's almost no opportunities to actually do this; the only major exception was whe

Also, Bitfinex almost always has a lower price than the other exchanges so there's almost no opportunities to actually do this; the only major exception was when the trustworthiness of Bitfinex's dollar deposits themselves were in question, which is a bad time to be doing arbitrage. Plus, you're going to be paying interest on your shorts for the entire time period they're open which eats into your profits. There's also the risk of the price increasing enough that a margin call liquidates your short position before the prices converge enough that you can close your position and take a profit.