We are one of the airports involved, we've never instructed aircraft joining the circuit or entering the ATZ to select Standby purely because its easier for us to keep track of them on the ATM, it also enables us to give positive traffic information to tcas equipped aircraft.
I suspect that this is the case at the other airfields who have SSR.
To myself and a lot of my colleagues, this is a pointless trial which could have been conducted 4 or 5 years ago by doing exactly what we (and others) have been doing all along, thereby achieving the same result which the CAA hope to achieve now.
