Just on PG & HG, I can say that 95% of all pilots actually only ever fly on the ridge from whence they started. Only a very small number will climb high enough to enable a cross country flight & in nearly all cases not in excess of 5,000` AGL, although recently, the UK pg height gain record was broken with a climb to something like 3,400 metres followed by a 40k glide - the exception that proves the rule.
Realistically, us lot are unable to transit controlled airspace to atco's requirements & thus are effectively excluded & so yes, we fly largely in the open FIR, transiting under & over aerodromes as required by current regulations. From my viewpoint therefore, transponders are costly in both weight & financial terms & unneccesary for the regulation of our activities.
I'm watching this thread with interest & I would like to see an argument in support of these things, in order that I may understand the CAA view - doesn't seem to be many participants in favour so far?