C J Driver,
You obviously know your stuff on this subject.
To broaden the question a little further if I may...
We are increasingly asked at major european airports - CDG, ZRH etc. to select mode S on the ground, for the benefit of ATC ground movement surveillance radar. This is a variation to our normal SOP of selecting SBY after landing, and selecting out of SBY normally to TA/RA when lining up for departure. As I understand it our equipment transmits mode S when in any mode except SBY.
The question is that if we were to change our SOP to leave the thing out of SBY when manoeuvering on the ground at all airports, would this still cause problems ( clutter on the radar), for other ATC units not equipped with such modern equipment?