ChrisN said: "Gpn, I don’t think you’re quite right about everybody needing it."
Unless it has an active element to it (e.g. it's own radar) then I don't see how it could work (typical example being Cessna flying with PCAS vs glider not equipped with anything). Happy to stand corrected on this. I do sense that PCAS/TCAS/ADS-B/FLARM means we're going to go down the 'Mode-S' thread again!