As I understand it, TWS's SMART routing feature is supposed to get you the best possible price that is available at all exchanges and ECN's. Can someone please explain: 1. When would someone not want to use SMART? If SMART will get you the best possible price, why would someone manually select ISLAND or any other ECN? 2. I see that TWS now allows you to exclude ARCA from SMART routing. I have seen some posts here about it but I still am not sure why people would want to do that. Thanks in advance.