When flying in OPEN FIR, presumably the Radar ADVISORY Service is such that they can only inform you about other traffic that the unit is either in contact with or which is painting a trace on their radar screen ? Therefore they can only give advice and not provide a completely controlled environment for any pilot who's IMC and requesting RAS ?
But now the argument gets circular
How can your "in cockpit" system show the target, but it is not on ATC's radar screen?
1 worrying answer - people rush out and buy FLARM, and not a Xpdr. Were we to get a drift to FLARM, we now end up with 2 incompatible ACAS systems. So (assuming the sytems perfect) Xpdr don't hit Xpdr, and FLARM don't hit FLARM, but FLARM and Xpdr hit each other?
It is I suppose an argument against FLARM - we have a number of ACAS "systems" based on Mode C, and any further systems should at least use that as a starting point?
NoD