Yes you may be right that the pilot was on the line on the map. But the point still is valid, a low performance aircraft ( and I include anything that is a light twin or single engine piston in that ) which is operating with those tolerances in the weather conditions stated could very easily enter the airspace before remedial action could be taken. Then were would the pilot be? Is it really that important to excercise your right to fly on the line of CAS when there is no need in poor weather conditions?
If the pilot was a professional or very experienced PPL what example does it set for less experienced pilots?
I would agree that a pilot who can fly accurately (which means a GPS) right on the line is equally capable of doing so a mile out, so flying right on the line is pointless and probably just gets the CAS owner nervous.
But it is possible that you are unaware of just how accurately one can fly today, in a modern IFR plane. I could fly a 900nm route, both ways, and the GPS tracks will be maybe 10m apart. That track log is from a GPS that receives EGNOS so its accuracy is a lot better than 10m. Obviously this is on autopilot.
On a number of occassions I've been warned of being in CAS when I was a couple of miles out, and either the controller was bluffing or his radar was well out. A radar can be pretty good in distance but azimuth accuracy is much harder. I am thinking of Shawbury talking about Birmingham airspace so 30-40nm. I verified the distance to CAS with the ILS DME of the airport too. Plus the 3 GPSs.