One of the concerns we have about ARCA (part of our "Redi family") is that they do in fact have traders who use the ARCA "give up" on the ECN's. This came to light a while back when we began to experience bids and offers simply fading away, without trades taking place. The explanation was that, although ARCA is an ECN, it is also an exchange, so it has actual bodies making markets in some issues.
I am doing some more research into this, and will be having a meeting with the ARCA big wheels this month. I really don't care what they do, as long as we know what they are doing (so we can make our decisions accordingly).
I am not saying that these traders necessarily have access to the ARCA books and/or stops, but we'll see.