Any system that detects ADS-B is better than nothing because nearly all airliners have it. Can you imagine the consequences for GA in the UK if a light aircraft, microlight, gyro, helicopter or glider were to pile into an airliner?
How does one pile into an airliner? How does one avoid an airliner? I suspect with considerable difficulty, as it's more likely to be the airliner that does the piling-in. In my hang-gliding days my instructor's advice was that if a military jet came along, one should turn to show them your wing... the thinking being that the speed differential was such that there was nothing you could do to avoid a jet, but if they knew you were there, they might just be able to avoid you.
I imagine a transponder is a more useful tool for avoiding meeting airliners. Even in class-G, someone can usually see you on radar and separate you.
Not saying a system that shows you the airliners is useless... but I doubt it's a panacea either.