The key to all this is understanding what the various pressure settings mean and what they can or should be used for. RPS is for assuring terrain clearance whilst a local QNH is more useful for avoiding airspace or for ATC to separate traffic in the area. The bottom line is that the pilot should use the most appropriate pressure setting(s) unless specifically instructed to use a different setting by ATC. The real problem arises when you're trying to avoid terrain and airspace at the same time!!