Beagle,
I take your point about the RPS being terrain safe, but there aren't many places in the UK where you are that far from an airport that can give you an accurate QNH.
The problem that I have with people flying around on RPS (especially when they are VFR) is that it causes an unbelievable number of unwitting airspace infrtingements. People, often talking to the military will skulk underneath the base of the class D airspace where I work aiming to be not above 1500' (the base being 1500')-they are quite entitled to do that (although I would rather they called) but because they are using the RPS they are usually 300 feet higher than they think they are as the base of the airspace is set according to our aerodrome QNH. Bosh they are actually at 1800', well inside the airspace and we have to take avoiding action with our passenger jets (assuming they've got mode C-if not they will be assumed to be below 1500 so the traffic will go over them by 200 feet at best!!)
You see my point??