Trader J/Babak,
Routing direct to SS would certainly makes sense if there were no ECN liquidity available. However, if there was, why not pay a little more in Commish for a better/quicker fill. ARCA has direct connections with at least 3 other ECNs whereby if there is contraside liquidity in their books, ARCA goes after it directly and bypasses the SNET pref route.
Additionally, as I have seen SS stocks trade, I have noticed a greater tendency for ECN's to dominate the inside price levels as MM's stay away assumingly for fear of the o second interval delay and SS executions against reserve volume. THus, if you enter a SS order while only ECN's are at the insede, the order is held by NAZ waiting for a MM to join or the ECN(s) to fill and move.
Plus the same old issues of not knowing how far back you are in the SS queue when there is momentum, versus ECN volume that you know is available as long its on the screen, etc. Just my thoughts as obviously we will all start to figure out the ramifications of SS now that it is fully live.