Spitoon, have to admit I never have been taught or seen what you say as happening. My experience in Oz and the Middle East has always been that below the transition level, all aircraft must always be on Area QNH, and above must always be on 1013. I knew that it was common practice for aircraft to switch to Area QNH when cleared to an altitude, but I thought that was a shortcut by the pilots that wasn't exactly Koshe. Since our radar system gives us Mode C based on the entered Area QNH into the system when aircraft are below the transition level, it will cause inaacuracies when separating between two aircraft, one that is correctly on Area QNH and the other that is incorrectly on 1013.
PPRUNE radar, ok, I got it the wrong way around, but we have had a reduction in separation occur here, purely because one aircraft switched to 1013 prior to leaving our FIR, due to the fact that 10,000ft in our FIR, becomes FL100 once crossed the boundary. Again I have to admit to never thinking that if you level off an aircraft to an intermediate FL after previously having cleared that aircraft to an altitude, that it is the ATC who has to ensure that the aircraft hasn't already switched to Area QNH. Since I was always led to believe that the aircraft should switch to or from 1013 on passing the transition level and not before, I never saw the need to ask the pilot which setting he was on.
As I said before, there seems to be a lot of different procedures operating throughout the world, and it would seem to make sense to me that all countries operated with the same rules and the same transition level. Then again, I would also think that everyone could operate with either hectopascals or inches but that's all too hard I guess.