The issue with BTC Spreads is transaction time. If you send BTC from one exchange to the other, most require at least a 6 block confirmation as of this morning that time sits at around 9 minutes. A lot of volatility can change in the hour that will take. If the transfer was instant, or you had a fast intermediary currency you may be able to beat the spread, but even a 500 dollar difference in bitcoin is only 2.8%. So you need to be able to beat the fees, and hope that the exchanges don't catch up to each other within an hour. Otherwise you lose out on the fee portion instead of materializing a gain.