ChrisN
That product doesn't give you direction info, so you have no idea where to look, where to turn, so it is basically impossible to act on the info especially if in IMC.
But more fundamentally, so many people fly with no transponders (sometimes intentionally OFF) that the present value of TCAS (outside CAS) is minimal. You still have to keep the same lookout.
Baloons often don't show on radar, BTW (I discovered this recently with Farnborough RIS).
It strikes me that in the absence of traffic information /weather upload, the pro-modeS argument is basically a pro-ModeC argument. And how far has Mode C got? After god knows how many years there are still INSTRUCTORS who teach it should be OFF outside CAS, etc. I am not saying Mode S is justifiable for GA but it's hardly suprising that a transponder will become compulsory eventually.