if it wasn't compulsory, would you turn yours on?
Despite the requirement for a transponder in class E airspace, and the requirement that if a serviceable transponder is fitted it "must" be used in any airspace, and including the obvious- (there are no non TSO'd transponders), mine always has been, and will continue to be turned on in any airspace when I have one fitted to the aircraft I am flying.
I am at a loss however with the ADSB requirement for a capability to turn that gadget off. Why have a switch? Why don't we all get a microchip implant in our heads to prevent the "rouge" VFR- PPL "idiots" from being let loose amongst us?
Come to think of it, why do transponders have a switch?