Example:
Last weekend I flew from Gloucester to Caernarfon on a direct track. Took off using the Gloster QNH, encountered lowish cloud and went 'on top' at FL65. Shawbury, Llanbedr and Valley were all closed, so no QNH available but we were able to set the RPS (obtained from London Info) and start our descent safely over an area of known high ground. Same applied on the return leg, when the cloudbase was lower still.
I agree that the use of multiple pressure settings can confuse students but altimetry isn't rocket science. If the tricks of the trade were taught better, there'd be fewer CFIT accidents!!
(BTW Caernarfon is lovely, beautiful mountains to fly (safely!) over and 2 minutes from the 5 mile stretch of golden sandy beach. Nice little museum and first class all day breakfast)