bookworm,
It's not something I know a great deal about but by the time an aircraft crosses an FIR boundary it's going to be FL 280+, is there much uncontrolled VFR traffic around at cruising levels? Or do you mean that in other FIRs the aerodrome approach phase of a flight is not conducted in a known traffic environment?
Can I put the question more simply, when I fly with a commercial airline on a scheduled flight, around Europe (or the USA?), how much confidence can I have that I'll be in Class D or better?